Bertrand Russell's blue spectacles analogy to describe the work of Kant  

"If you always wore blue spectacles, you could be sure of seeing everything blue (this is not Kant's illustration). Similarly, since you always wear spatial spectacles in your mind, you are sure of always seeing everything in space. Thus geometry is a priori in the sense that it must be true of everything experienced, but we have no reason to suppose that anything analogous is true of things in themselves, which we do not experience. Space and time, Kant says, are not concepts; they are forms of "intuition." (The German word is "Anschauung," which means literally "looking at" or "view."

The word "intuition," though the accepted translation is not altogether a satisfactory one.) There are also, however, a priori concepts; these are the twelve "categories," which Kant derives from the forms of the syllogism. The twelve categories are divided into four sets of three: (1) of quantity: unity, plurality, totality; (2) of quality: reality, negation, limitation; (3) of relation: substance-and-accident, cause-and effect, reciprocity; (4) of modality: possibility, existence, necessity. These are subjective in the same sense in which space and time are--that is to say, our mental constitution is such that they are applicable to whatever we experience, but there is no reason to suppose them applicable to things in themselves. As regards cause, however, there is an inconsistency, for things in themselves are regarded by Kant as causes of sensations, and free volitions are held by him to be causes of occurrences in space and time. This inconsistency is not an accidental oversight; it is an essential part of his system."