To this point in the course, we've described reason as the faculty that, from the onset of and without any help from experience, is able to combine two given ideas. Now it's time to ask where we get these ideas. All ideas have the same subject – a phenomenon. This was demonstrated by the earlier definitions, but it might have been anticipated a priori by the following reasoning. Necessary propositions state the conditions to which experience is subject. This means that every necessary proposition has to contain two terms – the part of experience with which the proposition is concerned and its conditions. Hence, all rational judgments have the following form: “Phenomena of such and such a kind are subject to such and such a condition.”

Of the two ideas that make up a rational judgment, therefore, one has an origin that we already know – experience. But whence comes the other? Its origin must be outside of experience, for otherwise it couldn't be related to experience. So these are a priori ideas (also called rational or first ideas). More specifically, they're the ideas of time, space, substance, cause, and end. Explaining their presence in the mind, Kant conceived of them as “determined forms,” as molds whose forms are taken on by phenomena as they are perceived. The mind simply takes note of this process and, when it has done so a certain number of times, concludes that: “All external phenomena are subsumed under the concept of space.”