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Can You Perceive a Star That’s No Longer There?

Michael Egnor

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At Uncommon Descent, V.J. Torley has a superb post on the question of objects of perception. He raises a profound question: If perception of an object entails a genuine encounter with the object, how can we perceive a star that we know does not actually exist now? He uses the example of a supernova in a distant galaxy — we see the star (a supernova), but we know, from astrophysics, that the star has actually disintegrated, and the only reason we perceive that it still exists is that the light from it takes a long time to reach us. If perception of an object is an encounter with the object, how can we perceive a star that doesn’t actually exist at the time we perceive it? It’s a fascinating and important question.

He raises an interesting possibility — that the object of perception may reach out to the subject, rather than the other way around. It has some explanatory power, to be sure, but issues can be raised: If perception still takes place at the sensory organ, one has no claim to knowledge about the external object, whether it reaches out or otherwise. Also, Aristotle’s notion of perception entails raising potency to act in the subject, not the object. “Reaching out” by the object seems to involve a similar motion in the object, which is difficult to understand in an inanimate object. It still seems to me that perception occurring (or at least beginning) at the object is the right way to understand it.

I’ve had some thoughts regarding Torley’s incisive thought problem — how perception of a supernova is to be understood when the distant star no longer exists. How can perception be squared with a relativistic universe, in which superluminal velocities are not permitted?

First some preliminaries. Perception must be an encounter with the object perceived. The Cartesian/Lockean view that we perceive only events in our sense organs and brain (that are stimulated by external objects) is untenable, because it is a reductio ad absurdum. If we directly perceive only objects within our skin, no knowledge of the external world is possible, and no science is possible — inferences about the real world are untestable. Even knowledge of our own mind is limited. We can’t use any empirical knowledge to understand ourselves. We can only use introspection. If we take the Cartesian/Lockean view seriously, “I think therefore I am” is the end of knowledge, as well as the beginning.

Chesterton appreciated the irony in solipsism:

A man wrote to say that he accepted nothing but Solipsism, and added that he had often wondered it was not a more common philosophy.

One of Torley’s commentors (Mapou) endorses the Cartesian/Lockean view:

This is like saying that any textual or video knowledge we have of a foreign country that we have never set foot in, is not knowledge because we do not perceive the country directly.

Mapou confuses knowledge in the sense of intellection with knowledge in the sense of perception. We certainly can intellect things we have never physically visited — I understand the concept of the number 4, although I have never visited the number 4 (having no location, the number 4 would be difficult to visit anyway). But I can’t perceive an object without some kind of physical encounter with it. To use Mapou’s example, I can’t perceive France unless I visit France. I can intellect France — I can conceive of it with my intellect without visiting it, but I cannot perceive it unless I encounter it in the real world. In the Aristotelian paradigm, when I perceive France, I become, in a sense, a Frenchman. France becomes an aspect of who I am. Certainly I can only perceive France — I can only become a Frenchman, in the Aristotelian sense of grasping the form of France — by visiting France. One doesn’t become a Frenchman by reading a travel brochure.

Furthermore, if perception of France does not entail actually encountering France, then we have no way to know if France actually exists. If perception is not of real objects, how can we know anything outside of our skin? Inferences are meaningless if they cannot be tested against reality. How can one distinguish France from Atlantis, if perception is merely of things inside our skin?

So, solipsism being ruled out, let’s return to the question of perceiving a star that’s not there anymore. From a Cartesian view, all I’m seeing from the star is its light. The actual star is gone. So how can my perception of the star be of the star, if the star is gone, and only its light remains? I believe that there is a satisfactory answer to this paradox, within the Aristotelian paradigm.

We begin by asking: how do we know the star is no longer there? With Aristotle, there are two ways of knowing: perception and intellection. Perception entails a grasp of the sensible species (sensible form) of the object, and intellection entails grasp of the intelligible species. ‘Sensible’ and ‘intelligible’ may be distinguished roughly: perception is knowing that an object is; intellection is knowing what an object is. When we know a star, we use both ways of knowing. We perceive the star (we see that it is by our sense organs), and we intellect the star (we understand what it is by our reason).

So how do we know that the star of a supernova is not there? We certainly don’t know by perception — in fact, we can’t know the star is not there by perception, because such information cannot be transmitted faster than light. So the only way we know the star is not there is by intellection — we understand, by astrophysics, that the evolution of a supernova is such that the star ceases to exist in a shorter period of time than the time it takes for the light of the star to reach us.

So we genuinely perceive the star, even if it has exploded in a distant galaxy, because perception is bounded by relativistic constraints. We cannot perceive objects beyond the relativistic constraints of distance and time. By perception, the star does exist. By intellection, the star does not exist. Aristotle insisted, contra his predecessors (and many successors), that perception and intellection are different modes of knowing, and they do not lead to the same knowledge (cf. DA III.3,427A21).

In this sense, the strangeness of relativity theory is confirmation of Aristotle’s insight. Until Einstein, our knowledge of reality (as regards constraints imposed by the speed of light) was perceptual. Einstein showed that intellectual knowledge of reality was different knowledge than perceptual knowledge of reality. What we know by intellection about what happens at distances is not what we know by perception. Perception and intellection are ontologically different knowledge. Neither is privileged.

And of course this dichotomy of perception and intellection regarding distance applies to all distances, not just interstellar distance. All of our perception is constrained by relativistic considerations — it takes light a measureable, if infinitesimal, time to reach us over inches, as well as over light years.

Another way of looking at this is to say that perception is constrained by time but not distance. Perception is always of now, but is unconstrained by distance. Intellection is not constrained by time or distance. Our object of intellection can be anywhere and anytime. We can intellect the near and the far — that a rose is on our desk, and that a supernova light years away no longer exists. We can intellect the past and the future — the moment of the Big Bang and the end of the universe.

We perceive things inside time, in the present. We intellect things outside of time. Perception is material. Intellection is immaterial. Perception is constrained by relativistic considerations. Intellection is not.

That is why we perceive a supernova, although we know it’s no longer there.

Image credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt.

Michael Egnor

Senior Fellow, Center for Natural & Artificial Intelligence
Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery, and award-winning brain surgeon. He was named one of New York’s best doctors by the New York Magazine in 2005. He received his medical education at Columbia University College of Physicians and Surgeons and completed his residency at Jackson Memorial Hospital. His research on hydrocephalus has been published in journals including Journal of Neurosurgery, Pediatrics, and Cerebrospinal Fluid Research. He is on the Scientific Advisory Board of the Hydrocephalus Association in the United States and has lectured extensively throughout the United States and Europe.

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