Immanuel Kant (1724-1804)
Hume's reflections on the origin of the idea of causation seem to show that the empiricist idea of the mind as a tabula rasa will not do. The mind is not just a passive receptor of sensory information. It does something with sensory input. One of the big questions for Kant is, just what is it that the mind does? He also wants to know where Hume's skeptical reasoning about the powers of human reason leave philosophy. In particular, Kant wonders, is metaphysics possible at all? That is to say, are we just wasting our time if we try to discover a priori truths about the nature of reality?
To help answer this question, he divides truths (or judgments about the world) into two kinds:
a) Analytical judgements merely analyze or explicate a given concept, adding nothing to it. For example, saying that bachelors are unmarried adds nothing to what we already know about the world, it merely defines (in part) the meaning of the concept bachelor.
b) Synthetical judgments, on the other hand, do not just analyze concepts. They add concepts together (synthesizing them), as in the judgment that bachelors watch more sports than married men.
Empirical judgments, such as scientific discoveries about the world, are both a posteriori and synthetic. But mathematical judgments, Kant says, are a priori and synthetic. Without going out and looking at the world, just by thinking (i.e. a priori), mathematicians are able to discover (synthetic) truths about the world. For example, Kant says:
"That a straight line is the shortest path between two points, is a synthetical proposition. For my concept of straight contains nothing of quantity, but only a quality."
If mathematics can discover synthetic truths a priori then perhaps there is hope for metaphysics.
BUT, how can such knowledge be possible? How can I have ideas of pure geometric shapes (things which, as Plato and Descartes pointed out, we never could experience in the physical world)? How can I know with certainty that the laws of arithmetic and geometry will always hold true? After all, these laws tell us about the physical world, which is why they are so useful to engineers. So why can't experience of the physical world teach us that these laws are wrong? Why is that impossible, inconceivable?
Because, Kant suggests, they are laws of what we can conceive. Mathematicians find out what is certainly true, true beyond doubt, by finding the limits of what the human mind can conceive or make sense of. If anything 'out there' beyond the mind breaks the laws of mathematics, we will never experience it as such because it will be beyond our minds' radar. Therefore it will never be part of our world, the world as we know it.
What we experience depends on what is out there, but how we experience it depends on our minds, on how they do what they do (which is, roughly, convert things into objects, events into experiences, data into meaningful, comprehensible information).
How do we experience the world? Most fundamentally we experience the world as spatial and temporal. Our world, the world as we experience it, is (with regard to objects) spatial, three-dimensional, geometrical. It is also (with regard to events) temporal, sequential, arithmetical.
According to Kant, "all bodies, together with the space in which they are, must be considered nothing but mere representations in us, and exist nowhere but in our thoughts."
How can this be? Because without the mind there is no experience, there can be no experience. So all experience, all that we experience, must be within, or through, the mind. Whatever is out there must be processed or filtered through the mind before we can experience it. And what we experience is then not the pure, raw external thing, but a processed (or shaped or interpreted or explained or filtered) object of experience.
This is a kind of idealism (remember Berkeley) but Kant calls it transcendental or critical idealism to distinguish it from Berkeley's theory. Berkeley held that a stone, say, is nothing but a collection of ideas. Kant says it is a mind-processed representation or product of something that really exists 'out there' beyond the mind.
Just in case Kant's ideas don't quite click yet for you, think of physics. Physics looks for, and finds, laws of nature, i.e. ways in which all objects in the universe must behave. But experience only tells us how the objects we happen to have experienced so far happen to have behaved (this is part of Hume's point about induction). A true law would apply to every possible object of experience. And it does indeed seem as though we know such laws, and know them for certain. But how can this be?
Because the most fundamental laws of physics are actually laws concerning how the human mind works, what kind of product it can have given its structure and functions. The raw data that the mind processes Kant calls noumena (singular: noumenon), and this data once processed by the mind he calls phenomena (singular: phenomenon). We cannot know anything at all about noumena, because we cannot know without the mind. It is as if the information in the external world must first be translated into the language of the mind before we can do anything with it, like sentences in English being converted into 1s and 0s inside a computer. Or as if the raw ingredients of the world must first be turned into something we can digest before we can take them in.
Nature (what science studies and where we think of ourselves as living) is the totality of all phenomena, of all actual and possible objects of our experience. It is the product of the interaction between noumena and our minds.
Kant says of the fundamental laws of physics and mathematics that: "The understanding does not derive its laws (a priori) from, but prescribes them to, nature."
The mind then is an object that processes data or input (which should not be thought of as already constituting anything as intelligible as information). By studying the limits of what we can think or make sense of, we can discover features of this processing. These may or may not be features of the noumenal world, but they will inevitably be features of the phenomenal world. The mind, Kant says, is such that it processes noumena into a world of objects in space interacting causally in time. This is our world, the phenomenal world, and causation will always, necessarily, be a feature of it. Asking how we know all events in future will have a cause is like asking how we know that all Krispy Kreme doughnuts will be round. That's how the processing machinery is set up.
It is pretty standard in both psychology and philosophy today to think of the mind as being somehow like a computer, and therefore as a data processor. So Kant's ideas might seem about right. But some people disagree with him. Consider how well Kant could answer the following questions:
1. Are the allegedly synthetic a priori truths he discusses genuinely synthetic (and not analytic)?
2. If we cannot know anything about the noumenal world, how can Kant talk about it so much?
3. If the mind is a kind of object, why does it never break down? Computers, for instance, break down. True, minds go wrong in various ways, but do they ever (short of just dying or going unconscious completely) stop doing the specific things Kant says they do? Why can nobody's mind conceive the things Kant describes as unthinkable (i.e. violations of the laws of arithmetic, geometry, and physics)?
4. If the mind is a kind of object, why do we all have exactly the same kind? Is it just a coincidence? After all, there are different kinds of computer, doughnut-maker, etc..
5. How significant is it that, according to modern physics, there are more than three dimensions (unthinkable and impossible, according to Kant) and the shortest distance between two points is not always a straight line?