What the reality of purely possible things is we shall discuss later on. Actually existing things at all events we assume to be _given_ to the knowing mind, not to be _created_ by the latter. Even in regard to these, however, we must remember that the mind in knowing them, in interpreting them, in seeking to penetrate the nature of them, is not purely passive; that reality as known to us—or, in other words, our knowledge of reality—is the product of a twofold factor: the subjective which is the mind, and the objective which is the extramental reality acting on, and thus revealing itself to, the mind. Hence it is that when we come to analyse in detail our knowledge of the nature of things—or, in other words, the natures of things as revealed to our minds—it will not be always easy to distinguish in each particular case the properties, aspects, relations, distinctions, etc., which are _real_ (in the sense of being there in the reality independently of the consideration of the mind) from those that are merely _logical_ (in the sense of being produced and superadded to the reality by the mental process itself).(67) Yet it is obviously a matter of the very first importance to determine, as far as may be possible, to what extent our knowledge of reality is not merely a mental _interpretation_, but a mental _construction_, of the latter; and whether, if there be a constructive or constitutive factor in thought, this should be regarded as interfering with the validity of thought as representative of reality. This problem—of the relation of the _ens rationis_ to the _ens reale_ in the process of cognition—has given rise to discussions which, in modern times, have largely contributed to the formation of that special branch of philosophical enquiry which is called Epistemology. But it must not be imagined that this very problem was not discussed, and very widely discussed, by philosophers long before the problem of the validity of knowledge assumed the prominent place it has won for itself in modern philosophy. Even a moderate familiarity with scholastic philosophy will enable the student to recognize this problem, in a variety of phases, in the discussions of the medieval schoolmen concerning the concepts of matter and form, the simplicity and composition of beings, and the nature of the various distinctions—whether logical, virtual, formal, or real—which the mind either invents or detects in the realities it endeavours to understand and explain.