Reliability of Sense Perception

Have you ever considered the effect your sense of touch, taste, and smell have on your spiritual life? I cannot have any thoughts about you or about the world or anything outside of my mind except through my senses. So as imperfect as my senses may be, that's the only avenue I have to reality outside of myself. But do our senses provide us with information perfectly? If not, how do we know what's true? Welcome to Renewing Your Mind on this Saturday.

I'm Lee Webb. We're making our way through Dr. R.C. Sproul's overview of classical apologetics, and today R.C. will help us understand the logic behind the reliability of our senses.

Can we trust what we touch and see? We continue once again now with our study in Christian apologetics, and we're in the middle now of looking at those elements of knowledge of the science of epistemology that are essential to coming to a sound defense of the truth of theism. And we've looked first of all at the law of non-contradiction, and in our last session we looked at the issue of the law of cause and effect or the law of causality, and we were careful to show that the proper definition is important to this whole discussion, that the law of causality is defined in this way, that for every effect there must be an antecedent cause. Now, today I said that we would look more carefully at the critical analysis of causality that was offered by the eighteenth-century Scots philosopher David Hume.

David Hume, in his inquiry into these matters of causality, made some observations about cause and effect that are very important. Under his analysis, he said this, that what we observe when we see things happening around us are what he calls relationships that are customary, customary relationships, or another term that he uses was relationships of contiguity. You know that if you have a house and you have a property line that the house next door may be said to be contiguous to your property because it abuts right up next to it.

So that which is next to something else would be contiguous to it. So what David Hume is saying here is that we see events transpire or take place in the external world that one thing follows another, and we assume that the one thing causes that which follows it because they follow in sequence on a regular basis. We see, for example, on a regular basis that it rains and when it rains the grass gets wet, so that there is a customary relationship between raining and wet grass. And this happens in such regular sequences or intervals that we come to the conclusion that the cause of the grasses becoming wet is the raining that precedes it. Now you look at me and you say, somebody really is raising a question of whether the rain is causing the wet grass. Everybody knows that the rain is that which causes the effect of wet grass. Well, in ordinary experience that's the way we think because that's the way it certainly seems to us under the naked vision that we have in our observations and because we are accustomed to thinking in this way. But how do we know that in between the falling of the rain and the dampening of the grass some invisible cause other than the rain is interceding and is the real cause for the grasses becoming wet?

Now, again that may seem like utter stupid assumptions. That's like speculating about men made of green cheese on the backside of the moon and so on. But from a philosophical perspective, particularly in light of the 17th and 18th century when philosophers were doing a penetrating analysis of understanding external reality and the forces that are in effect making happen what makes happen with the school of Descartes, with his theory of interactionism, and with other philosophers such as Spinoza and Leibniz in the rational school who postulated invisible causes that were not seen for that which you can observe empirically. And so there was a big controversy in science and in philosophy about actual causes.

See if I can make something of an illustration right now. What is it that makes us sick from time to time? Keep in view that now when we are ill or something we might go in and get blood tests done or throat cultures and these cultures are placed under microscopes and we begin to discover that organisms or antibodies are at work infecting our well-being that are invisible to the naked eye and that without the discovery of microscopes and that sort of thing we never would have imagined that the causes for our diseases are what they are presumed to be now. And it wasn't that long ago where people talked about animal spirits invading our bodies and going up and down our arms and so on before the use of the microscope. So again what the microscope has done is opened up a whole new world of realities that are truly there and are making an impact on our lives and they're flying around in this room right now that we can't even see. We say, well, we picked up a bug or a germ or so on.

I had the flu last week and somebody told me the second day of the flu that it was a 24-hour bug and I said, well, my 24-hour bug needs a new wristwatch then because he's still here. But we talk about bugs and we talk about these things that infect us but what Hume was doing from a scientific analysis was there's all kinds of things going on that we don't perceive. We don't ever really see and we make assumptions that because one thing follows another, it is therefore the cause. Now, causal thinking, ladies and gentlemen, is at the heart of the scientific inquiry. It's at the heart of medicine. When we're ill and we go to the doctor and we ask the doctor for a diagnosis, we're asking him to determine and isolate the cause for our illness and we want him to be able to determine the cause so that he can come up with a treatment that will cure the disease. But if we can't determine what's wrong and don't have a proper diagnosis, it's very difficult to find the proper remedy. And not only that, but in biology, in chemistry, in physics, in astronomy, the principle of causality is assumed constantly. To say it another way, causal thinking is at the heart of natural science.

Now, you can imagine the crisis that came about in the 18th century when this very learned scholar in Scotland raised questions about scientific ability to determine and isolate causes. Now, in his analysis of this, David Hume used a famous illustration, and the illustration was what we call the pool ball illustration. If you can imagine a pool table, and there's a pocket at the end of the table, and in the middle of the table you have the cue ball, and then you have the object ball, we'll call it the eight ball, and the object here is the pool player wants to sink the eight ball in the corner pocket. And so he picks up the cue stick, puts some chalk on the end of the stick, and he aims the cue stick at the cue ball. And then using the motion of his arm, he swings the cue stick and moves the cue stick. The cue stick strikes the cue ball, imparting force presumably to the cue ball, setting the cue ball in motion. And the cue ball moves across the table until it hits or collides with the object ball, in this case the eight ball, and then immediately following that, the eight ball begins to move, and if our aim and striking were correct and proper, ultimately the object ball is deposited in the corner pocket.

But you see all kinds of physical actions involved in that. I pick up the cue stick. I aim the cue stick. I swing my arm, and since my arm is connected to the cue stick, the cue stick moves. When I move the cue stick, I hit the cue ball with the cue stick, and then the cue ball hits the object ball, and I make the assumption that there is a causal relationship and sequence all along here. But does anybody actually perceive invisible force transferred from the end of the cue stick to the cue ball? Does anybody see the transfer of force from the cue ball to the object ball?

No. What you see is one thing following another, and that is what's called a customary relationship or a contiguous relationship. And what Hume is saying is you don't see causality.

You see several actions taking place in sequence, and you're assuming a causal nexus or a causal relationship from those distinct actions. You know, the old story of the farmer who was awakened every morning before he wanted to wake up because at the crack of dawn, the rooster crow. And he said, you know, every time that rooster mine crows, the sun comes up, and it gets in my eyes coming in this window, and it wakes me up.

And so if I want to stop the sun from coming up, I'm just going to strangle that rooster. He thought, I can kill the rooster, I can stop the sun, because every day the rooster crowed, then the sun came up, and he assumed that it was the rooster causing the sun to come up. And that's an example of a fallacy that we learn, an informal fallacy, in the science of logic called the post hoc ergo propter hoc fallacy, which being translated means the fallacy of after this, therefore because of this. That just because something happens after something else does not mean that the previous thing had anything to do with that which came afterwards. And you can't just assume because one thing follows another that they are connected causally. Now that's a brief overview of David Hume's analysis of the problems of causality. Now what happened was you have all kinds of people who have assumed, there's kind of a causal leap with their minds, who have assumed that what Hume did with this analysis was demolish the law of causality, which he did not do. I mean he actually himself came to the conclusion, since we can't know what is causing a particular effect, we might come to the conclusion that anything can produce anything else.

Now that's fine if he wants to talk like that. However, what he doesn't show is that nothing is causing the movement of the pool ball. It's one thing to say there are forces engaged here that I don't perceive, that I can't see.

It's one thing to say I don't know what's causing the motion, and it's a huge movement from that to say nothing is causing the motion. Again, when we apply the law of causality to the external world, we have to face the limitations of sense perception that David Hume sets forth. But we don't have to therefore jettison the principle of the law of cause and effect, because it still remains a fact that if something is in effect, it was indeed caused by something other than itself.

That has to be true no matter how many experiments you take, because again I remind you that is a formal argument. Now remember that the third principle that I talked about was the principle of the basic reliability of sense perception. And I labored the point that we admit that our senses do not have perfect perception of reality. That's why we have machines to heighten our ability to perceive things like telescopes and microscopes, as I mentioned before. And remember I mentioned the illustration of Augustine's bent oar in the water and somebody's head being the size of my thumbnail, because I can cover their head from a distance with my thumb, and understanding principles of depth perception, and that we understand there are limits to our powers of perception. Now what David Hume does here is shows those limits that we never can penetrate to the invisible realm where perhaps all kinds of unseen forces are at work, not the least of which is the power of God. Again, the New Testament tells us that in God we live and move and have our being.

Now just take that second part of that. In God we move. And one of the principle assumptions of Christian truth is that nothing can move in this world apart from the power of God. And I can say I have the power to drop this piece of chalk and make it move, but at best I'm a secondary cause, because I can't even open my finger apart from the power of God.

And the power of God is invisible. So the other side of the coin is, this is an argument that is compatible with Christian theism that says there can be no power without God, and that God is the power supply of all motion, and because He's invisible, no amount of empirical research will get to the heart of the matter, to get to the heart of matter, to get to the heart of the motion of matter. And so in that regard, I have no complaint with David Hume's analysis where he shows the limits of human sense perception. Unfortunately, he took it to this place where he tried to reduce all science that depends upon sense perception to skepticism. In fact, it is said of Hume that he represents the graveyard of British empiricism. Immanuel Kant, who came after Hume, said it was Hume's skeptical analysis of being able to perceive causal forces that awakened Kant from what he called his dogmatic slumbers, and Kant set about his whole rest of his career to rescue science from pure skepticism.

Because if you demolish causal thinking, it's not just Christian theism that goes, it's science that goes. And so Kant sought to resurrect the validity of causality and the basic reliability of sense perception. Again, my senses cannot give me a comprehensive view of reality, but the only link I have from the interior chamber of my mind, my thinking, to the external world, the only transition I have from the mind to you is through my senses. My body is the bridge from my mind to the world. What is mind?

Dr. Gerstner used to answer that question. What is mind? No matter. What is matter then?

Never mind. What he was getting at with his cute little game was that there is a fundamental distinction between materiality, corporeality, and that which is non-physical, and an idea, a thought in the mind may be linked to a physical cause of chemical simulations and synapses and so on in the physical surface of the brain. But it's one thing to say the physical brain gives rise to thinking. It's another thing to say that thinking itself is physical.

And we don't want to do that. Okay, so I'm saying is that my thinking, my conscious awareness of things is non-physical, but I cannot have any thoughts about you or about the world or anything outside of my mind except through my senses. So as imperfect as my senses may be, that's the only avenue I have to reality, physical reality, outside of myself. I can retreat into my own mind and make all kinds of deductions about what may or may not be out there, but I have no actual contact with the world out there except through my senses. That's why it's axiomatic in modern science and in biblical studies to operate with the assumption that our basic equipment that we have, the faculties of knowing that God has given us, such as seeing, hearing, tasting, smelling, touching, and so on, are at least reliable enough for us to act upon.

Again, when I see the light turn red, I can speculate that maybe a demon caused it or it's a malfunction and that's a possibility, but I trust it enough to put my foot on the brake. And this is the way the Bible speaks when it talks about, as I mentioned earlier, Peter saying, we don't believe in cleverly devised myths or fables, but we believe in what we have seen with our eyes and heard with our ears. The basic reliability of sense perception and even the assumption that we can see causal relationships in this world is assumed throughout Scripture. And so these three principles are non-negotiable to Christian apologetics. Again, the law of non-contradiction, B, the law of causality, and three, now, the basic reliability of sense perception. Basic logic.

It is our ally in defending the Christian faith. We're glad you joined us on this Saturday edition of Renewing Your Mind. I'm Lee Webb, and, you know, these are not simple concepts. It takes some diligent study to be able to use them effectively in defending our faith.

That's why we'd like to send you Dr. R.C. Sproul's 32-message series. In his overview of classical apologetics, he surveys its history and teaches us to defend the existence of God and the historical truth claims of Jesus Christ. The series is titled Defending Your Faith, and we will send it to you for your donation of any amount to Ligonier Ministries. You can call us with your gift at 800-435-4343. You can also find us online at renewingyourmind.org. We're also including a bonus disc that contains the MP3 audio files for the series and a PDF of the study guide. That guide has additional reading suggestions, study questions, and an outline of each message. So I do hope you'll reach out to us and request this series, Defending Your Faith by Dr. R.C.

Sproul. Our number again is 800-435-4343, and our web address is renewingyourmind.org. And in advance, let me thank you for your generous donation. You can learn more about apologetics and a Christian worldview when you download our free app. Whether you're at home or on the go, trustworthy teaching is always within reach. You'll have access to thousands of discipleship resources from Dr. Sproul, our Ligonier Teaching Fellows, and other trusted pastors and teachers.

To download the app, simply search for Ligonier in your app store. And as we conclude our time together, let me pose this question. Is it possible for us as finite creatures to say anything meaningful about the perfect, transcendent God? Can we describe Him with our words, or are we just describing our own subjective feelings? Next week, R.C. introduces the fourth basic building block for knowledge. So, we hope you'll join us for Renewing Your Mind. . .