To expand on
@The Great Whatever's concise answer...
I have trouble distinguishing between analytic and a priori for example. Do they just have the same meaning with 2 different ways of saying it or is there some other distinction? — ladyphoenix86
Good question. Some philosophers have believed analytic and a priori to be coextensive, and the same goes for synthetic and a posteriori. From this empiricist point of view, whatever is analytic is a priori and whatever is synthetic is a posteriori, and vice versa.
But they have different meanings. Analytic-synthetic is semantic, and a priori-a posteriori is epistemological.
Analytic-synthetic is about what makes a proposition true. Analytic propositions are true by virtue of what their words mean, and synthetic propositions are
not true merely by virtue of what their words mean. One of Kant's ways of thinking about the difference is that analytic truths don't tell us very much, i.e., they are
explicative, whereas synthetic truths can tell us something new, i.e., they are
ampliative.
A priori-a posteriori is about how we know things or how we justify our knowledge. A priori knowledge is known independently of experience. A posteriori, or
empirical, knowledge is known from experience. The thing to note about a posteriori knowledge is that because it is confirmed or disconfirmed by experience, it tells us what happens to be the case, and not what must be the case, i.e., this kind of knowledge is about what is
contingent. In contrast, a priori knowledge is neither confirmed nor disconfirmed by experience, and so concerns what must be the case, i.e., this kind of knowledge is about what is
necessary.
As you've noticed, some philosophers think there is synthetic a priori knowledge.
why is 'synthetic a priori' different to 'analytic a posteriori'? — ladyphoenix86
To know a synthetic proposition a priori is to know something that is not true merely due to the definitions of the terms involved, and to know it independently of experience too. This is important because, if such knowledge is possible, then we can have substantial, ampliative knowledge (from the synthetic component) that does not depend on experience, i.e., that we attain using our own reason unaided by experiential confirmation.
Analytic a posteriori knowledge, on the other hand, seems impossible. It doesn't make much sense to say a sentence that's true by virtue of the meaning of the constituent words can be known from experience. If you understand it then you know it's true already, without any perception or investigation of the world.
The main thing I haven't really addressed here is what "independently of experience" precisely means, because it can be interpreted strictly or loosely.