There is no definite way to confirm that we know anything at all. Only from our direct experience can we claim any knowledge about the world.
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The role of perception in knowledge
It is hard to imagine a world that exists outside of what we can perceive. In the effort to get through each day without crashing our cars or some other calamity, we make assumptions about the objects in our physical world. Their continuity, their behavior.
Some of these assumptions are based on our own experience, some on the knowledge imparted by others of their experience, and some on inferences of logic.
Experience, however, comes through the lens of perception. How things look, how they feel, how they sound.
Our understanding of, and interaction with, the world comes through particular constructs of the human body – eyes, ears, fingers, etc. Most people intuitively understand the subjectivity of some of our perceptions.
Colors look ‘different’ to people who are color blind. Our feeling of temperature is impacted by immediate contrast – People stepping outside the doors of an airport will have a different impression of the temperature if they have just come from Moose Jaw or Cancun.
Even more substantial understandings come to us through the lens of our senses. We can see the shape of a tree, or we could close our eyes and infer the shape through touch, but in either case, or even combining the two, we are relying on our senses to impart an understanding of the physical world.
The question of what objectively ‘is’, is something that has long been one of the subjects of philosophy. Philosophers from Descartes to Kant have tried to describe our existence in such a way as to arrive at understanding of the physical world in which things can be conclusively known.
Descartes introduces the idea in his Meditations: “Surely whatever I had admitted until now as most true I received either from the senses or through the senses. However, I have noticed that the senses are sometimes deceptive; and it is a mark of prudence never to place our complete trust in those who have deceived us even once.”
Descartes famously employed systematic doubt, questioning all knowledge conveyed by his experience in the world until the only knowledge he couldn’t doubt was the fact that he could doubt.
Therefore I suppose that everything I see is false. I believe that none of what my deceitful memory represents ever existed. I have no sense whatever. Body, shape, extension, movement, and place are all chimeras. What then will be true? … Thus, after everything has been most carefully weighed, it must finally be established that this pronouncement “I am, I exist” is necessarily true every time I utter it or conceive it in my mind. (Descartes, Meditations)
Descartes confirmed we have a self. Unfortunately this self could be the one we see in the mirror each morning or a brain in a vat. If the only thing we cannot doubt is that we can doubt, essentially that guarantees us having only the mechanism to doubt. No body. We could therefore be isolated brains, being manipulated by things unknown, our entire world a mirage.
How then can we hope to claim knowledge about the physical world?
For Locke, our understanding of the world comes from our experience of it. It is this experience that provides knowledge. He says, in his Essay Concerning Human Understanding:
Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas: – How comes it to be furnished? Whence comes it by that vast store with the busy and boundless fancy of man has painted on it with an almost endless variety Whence has it all the materials or reason and knowledge? To this I answer, in one word, from EXPERIENCE. In that all our knowledge is founded; and from that it ultimately derives itself.
He wrote that there were two types of qualities, ones that existed innately in an object or series of objects, such as size, number, or motion, and those that are wholly dependent on our perception of them, such as color or smell.
The particular bulk, number, figure, and motion of the parts of fire or snow are really in them, whether one’s senses perceive them or no: and therefore they may be called real qualities, because they really exist in those bodies. But light, heat, whiteness, or coldness are not more really in them than sickness or pain is in manna. (Locke, An Essay Concerning Human Understanding)
Experience then, as long as we have an understanding of the limitations of our perception, will confer certain truths about the physical world we inhabit. For example, through experience we can claim knowledge of how many crows are perched on a telephone wire, but not how many of them have ‘black’’ as an intrinsic property of their feathers.
Quite in opposition to this was George Berkeley (pronounced Bar-clay), for whom ‘to be’ was ‘to be perceived’. Berkeley wrote in A Treatise Concerning the Principles of Human Knowledge:
Besides all that endless variety of ideas or objects of knowledge, there is likewise something which knows or perceives them and exercised divers operations, as willing, imagining, remembering, about them. This perceiving … does not denote any one of my ideas, but a thing entirely distinct from them, wherein they exist or, which is the same thing, whereby they are perceived – for the existence of an idea consists in being perceived.
Because our knowledge of the world comes from our perception of it, it is impossible to conclusively know the existence of anything independent of our perception. Berkeley, wrote:
Hence, as it is impossible for me to see or feel anything without an actual sensation of that thing, so it is impossible for me to conceive in my thoughts any sensible thing or object distinct from the sensation or perception of it.
This line of inquiry ultimately results in the entire physical world being called into question, as Berkeley observed:
If we have any knowledge at all of external things, it must be by reason, inferring their existence from what is immediately perceived by sense. {However} it is granted on all hands (and what happens in dreams, frenzies, and the like, puts it beyond dispute) that it is possible we might be affected with all the ideas we have now, though no bodies existed without resembling them.
If we can not know things outside of perception, and our perceptions are entirely unreliable, where does that leave us? It certainly isn’t useful to imagine your existence as the sum total of your knowledge, or that our experiences are inherently mistrustful.
What these philosophies can be useful for understanding though, is that often what we consider knowledge is more of a general social agreement on a somewhat consistent comprehension of the things before us. For example, we appreciate that the color green can be perceived differently by various people, but we organize our language based on a general understanding of the color green without worrying about the particular experience of green that any individual may have.
For David Hume, there definitely was a physical world, our perception of which was ultimately responsible for all of our ideas, no matter how complex or abstract. He wrote in An Enquiry Concerning Human Understanding:
When we analyze our thoughts or ideas, however compounded or sublime, we always find that they resolve themselves into such simple ideas as were copied from a precedent feeling or sentiment. Even those ideas, which, at first view, seem the most wide of this origin, are found, upon a nearer scrutiny, to be derived from it.
Furthermore, since all of our perceptions of the physical world are coming from the same physical world, and the nature of perceiving works more or less the same in each person, we can achieve a consistency in our understanding.
So although it may not be possible to know things with the same certainty as knowing oneself, or to be able to really describe the construct of the world outside of our perception of it, at least we can get along with each other because of a general consistency of experience.
However, this experience still admits to a certain fragility. There is no guarantee that past experiences will be consistent with future ones. In An Enquiry Concerning Human Understanding, Hume observes:
Being determined by custom to transfer the past to the future, in all our inferences; where the past has been entirely regular and uniform, we expect the event with the greatest assurance and leave no room for any contrary supposition. But where different effects have been found to follow from causes, which are to appearance exactly similar, all these various effects must occur to the mind in transferring the past to the future, and enter into our consideration, when we determine the probability of the event.
To simultaneously understand all effects when considering an event in the future is not necessarily a limitation, thanks to our amazingly sophisticated brains. Immanuel Kant thought that the way we process the information provided by our senses was an important component of knowledge. Kant wrote in the Prolegomena to Any Future Metaphysics:
The difference between truth and dreaming is not ascertained by the nature of the representations which are referred to objects (for they are the same in both cases), but by their connection according to those rules which determine the coherence of the representation in the concept of an object, and by ascertaining whether they can subsist together in experience or not.
Kant did not support the view that the existence of objects was called into question because of the subjectivity of the perceptions by which we must experience them, but neither that all knowledge of the physical world comes from experience. Kant argued:
Experience teaches us what exists and how it exists, but never that it must necessarily exist so and not otherwise. Experience therefore can never teach us the nature of things in themselves.
Knowledge then, is made up of things we infer, things we experience, and the way our brain processes both. The great metaphysical question of ‘Why it is all this way?’ may always be out of our reach.
Understanding some of this metaphysical uncertainty in knowledge does not mean that we have to give up on knowing anything. It simply points to a certain subjectivity, an allowance for different conceptions of the world. And hopefully it offers a set of tools with which to evaluate or build claims of knowledge.