The _etcetera_, with which in that last passage Kant concludes his list of pure intellectual concepts, indicates a problem that must very soon have made itself felt. That it did so, appears from his letter to Herz (February 21, 1772). He there informs his correspondent, that, in developing his _Transcendentalphilosophie_ (the first occurrence of that title in Kant’s writings), he has “...sought to reduce all concepts of completely pure reason to a fixed number of categories [this term also appearing for the first time], not in the manner of Aristotle, who in his ten predicaments merely set them side by side in a sort of order, just as he might happen upon them, but as they distribute themselves of themselves according to _some few principles_ of the understanding.”[692] Though in this same letter Kant professes to have solved his problems, and to be in a position to publish his _Critique of Pure Reason_ (this title is already employed) “within some three months,” the phrase “some few principles” clearly shows that he has not yet developed the teaching embodied in the metaphysical deduction. For its keynote is insistence upon the necessity of a _single_ principle, sufficient to reduce them not merely to classes but to system. The difficulty of discovering such a principle must have been one of the causes which delayed completion of the _Critique_. The only data at our disposal for reconstructing the various stages through which Kant’s views may have passed in the period between February 1772 and 1781 are the _Reflexionen_, but they are sufficiently ample to allow of our doing so with considerable definiteness.[693] In the _Dissertation_ Kant had traced the concepts of space and time, no less than the concepts of understanding, to mental activities. “Both concepts [space and time] are undoubtedly acquired. They are not, however, abstracted from the sensing of objects (for sensation gives the matter, not the form of human cognition). As immutable types they are intuitively apprehended from the activity whereby the mind co-ordinates its sensuous data in accordance with perpetual laws.”[694]