When I turn my eye inward, I find only doubt and ignorance. Every step I take is with hesitation; every new reflection makes me dread an error and absurdity in my reasoning.
-- philosopher David Hume (1711-1776)
In order to find out which of our knowledge is certain, many philosophers start out by deciding how much of it is uncertain. In this way, doubt can be a powerful way to think more clearly about what we know.
Doubt is known in philosophy as "skepticism." While in common language, a skeptic might question someone's motives or logic, in philosophy, a skeptic might question whether the sky is blue.
Most philosophers start off right away by doubting the senses. It's pretty easy to do. I know that my senses sometimes mislead me, so assume for the sake of argument that they always mislead me.
An analogy might be helpful here. In the movie The Matrix, the character Neo discovers that the world he thought was real is actually a computer simulation. His sensory experiences seem real, but they are not. For the sake of argument, let's assume that all our sensory experiences are somehow wrong, like a computer simulation.
While this might seem bizarre and far-fetched, so does the relativity of time and space, and quantum physics. A feeling of certainty is proof of nothing. If we are going to be honest with ourselves looking for certainty, we must ignore feelings of certainty and look for the real thing.
What else can we doubt?
Time is full of misunderstandings and contradictions, so it's a good place to look. Modern physics teaches that the common-sense view of time is wrong. How do we know for sure that the past or future exists? Our only knowledge of the past is through memories. Even though memories are about the past, they occur now. That is, if I remember my first kiss, the kiss may have happened in the past, but the remembering occurs now. For all I know, I was created three seconds ago, with all my current memories thrown in for laughs.
Our only knowledge of the future is through expectations, which are even fuzzier than our memories.
So we can doubt that we have any certain knowledge of the past or the future. Any certain knowledge must lie in the present.
All our senses and memories may be wrong, but what about "absolute truths" like logic and mathematics? We can at least say that it is certain that a triangle has three sides, right?
Logic and mathematics are essentially a set of definitions. It is pointless to say that they are "true" or "false" in themselves, any more than it would be to say that the rules of chess are true or false. The question is whether logic and mathematics give us certain knowledge about something other than themselves.
Modern physics certainly calls into question whether the universe is logical from a human perspective. Wave/particle duality and the wave function's many contradictory worlds are only hints of the way the universe is beyond human logic. Our space-time world doesn't obey the rules of standard geometry either, and might be beyond any mathematics that the human mind can understand.
So we can doubt that even logic and mathematics are certain.
What about the idea that there is something in the universe besides ourselves? We are sure about that, right?
The idea that something besides ourselves exists comes from our senses, which we've already called into doubt. If we are going to be good doubters, we must hold open the possibility that all our experiences could come from inside us, as if we were dreaming them. (Your dreams do come from inside you, right?)
The philosophy that I am the only thing in this universe is called "solipsism."
The philosopher Descartes followed a line of reasoning like the above, and came up with his famous line, "My head hurts." Actually, what he said is, "I think, therefore I am."
In other words, all my thoughts and experiences may be wrong, but at least I know that I am the one thinking and experiencing them.
This extreme doubt, doubting senses, memories, past and future, logic and mathemetics, even the existence of anything outside myself, leaves one certain fact: I exist; I am the one experiencing these questionable things.
But hold on a minute. Is even this fact certain? Has no one managed to doubt it?
As you might suspect, even the existence of self has been called into question. Siddartha Guatama -- the Buddha -- started one of the world's major religions around the principle that what appears to be self is not. The philosopher David Hume also managed to question whether our idea of "self" has any support.
We now arrive at the most extreme of doubt: We cannot say that any knowledge is 100% certain. Has this philosophizing degenerated into silly mental play, with no room left to grow? Is it all we can do at this point to throw up our hands and leave all this doubt as irrelevant to real life? Where do we stand?
Next: The Ground Floor
For Further Exploration
The philosophically minded and curious can browse these sources elsewhere on the Web:
- Descartes' Meditations on First Philosophy: the source of the famous "I think, therefore I am"
- Hume's An Enquiry Concerning Human Understanding: a classic exposition of skepticism and epistemology
- "Responding To Skepticism:" selection from a book on skepticism
- Skepticism: an overview of skepticism in philosophy