<human>:  When Kant says that the categories are a priori, what does he mean?
<bot>: The categories are the basic concepts of the mind. They are the building blocks of all knowledge. Kant says that the categories are a priori because they are not based on experience, but rather on our ability to think. For example, Kant argues that the categories of space and time are a priori because we can never directly observe space or time.