Since I critiqued rationalism earlier (here), it’s only fair now to go after its traditional rival, empiricism.
In the case of rationalism, there is an official doctrine explicitly held by rationalist philosophers, and then there are habits of thinking and implicit assumptions that are likely to be found among rationalists. It is the latter that I attacked.
In the case of empiricism, there aren’t really distinctive habits of thinking that are especially common among empiricists. The error just lies in the official, explicitly held doctrine of empiricists. What is that doctrine?
Empiricism: The view that there is no synthetic, a priori knowledge. Or: All substantive knowledge (about objective reality) rests on observation.
Synthetic claim: A claim that is made true by facts in the world, rather than merely by the meanings of words; a claim that is not analytic.*
Analytic claim: One that is true by definition.*
A priori knowledge: Knowledge whose justification does not derive from observations (including the five senses and/or introspection).
[*There are other ways of defining “analytic” and “synthetic”, but I don’t want to argue about that distinction now.]
The empiricists’ error is that of thinking in an overly rationalistic manner. That is, instead of looking at actual examples of knowledge and generalizing from there, the empiricists lay down one or more a priori principles about knowledge and deduce that all examples must conform to those principles. Any examples that don’t seem to fit must be illusory.
I know one argument for empiricism; it goes something like this. In order for a person to count as knowing some proposition, it is necessary that the person be right about that proposition, and that this not be due to chance. However, if we were to have a priori beliefs about the world, these beliefs would not be caused by any interaction we had with the things the beliefs are about. In the absence of such a causal connection, it would be sheer luck if the content of our beliefs happened to match the facts about those things. So these beliefs could not be knowledge.
It’s okay for us to have analytic a priori knowledge, though, because this (according to the tradition) is really only knowledge about the meanings of words, or about the relationships among our concepts, and both of those things are dependent on us. The problem only concerns knowledge of facts that are supposed to be independent of our minds.
Notice how the above is an a priori argument for empiricism. Empiricists might try to reconcile this by (falsely) claiming that empiricism is analytic. But don’t worry; my criticism isn’t that empiricism is self-defeating. My criticism is that empiricists decide what knowledge has to be like first, and then look at the examples and try to force them to fit their assumptions.
There are many well-known counter-examples to empiricism. E.g.:
Those are all, apparently, synthetic statements that can be known a priori. Take the last claim: “Nothing can be completely red and completely blue.”
So it’s synthetic a priori. Similar remarks could be made about the other examples, and many more besides.
This all looks to me fairly straightforward, which makes the popularity of empiricism during the last three hundred years somewhat baffling.
I know what really happened, though. Besides the a priori argument for empiricism, what really motivated empiricism during the last 300 years was scientism – a sort of ideology that revolves around efforts to sound “scientific” (under a simplistic interpretation of what that means) and to express exaggerated reverence for science. In the last 300 years, a simplistic interpretation of the history of science has been popular, according to which the key to scientific progress from Galileo on was the reliance on observations and the rejection of a priori intuitions.
People started competing with each other to see how extreme they could make their science-worship. (You know, sort of like how today’s progressives are falling all over themselves trying to find ever more extreme professions of “anti-racism”.) Since modern science was held to be based entirely on observation, victims of scientism decided to declare that all valid knowledge must be so based. Anything that’s not like science is complete garbage.
This led to a bunch of people in the 20th century, continuing through today, rejecting free will, rejecting the reality of consciousness, rejecting the whole field of ethics, etc. Needless to say, these are insane positions.
Also needless to say, this is a completely unscientific way of thinking. The scientific method, on any account, does not include taking up ideological commitments based on your desire to express a worshipful attitude toward something (however good that thing might be), then trying to shoehorn all phenomena that you encounter into the mold created by those ideological commitments. I’m sure the empiricists won’t admit that that’s what they’re doing. But it is.
Here, though, an older and wiser A.J. Ayer admits that his philosophy was false: https://www.youtube.com/watch?v=nG0EWNezFl4.