All problems are merely subjective; they are a conflict between two conceptions and as Professor Mach himself says, the solution of problems consists in the adaptation of thought to facts, i. e. to new facts or new views of facts. By an adaptation of our thought to the enlarged field of vision the problem vanishes; it has ceased to be a problem. In fact it never existed as an objective phenomenon. There are no problems in nature. There are problems only to the investigating mind. But even the formulation of problems is a problem to be solved, and perhaps the most difficult and subtle kind of problems is to discover the flaw in wrongly formulated problems. The problem of the duality of body and soul, matter and mind, feeling and motion, ceases to be a problem to him who has worked his way through to a monistic conception, but to those who have not as yet succeeded in establishing a unitary view of these ideas, because they take them to be separate and distinct existences, it is a problem of great importance. XII. THE ONENESS OF SUBJECTIVITY AND OBJECTIVITY. The world is not rigid being, but activity, not absolute existence but a system of changing relations, not an abstract _Sein_ but a concrete _Wirklichkeit_—a constant working of cause and effect. There is no dualism in this, for the _Wirklichkeit_ is one and undivided. Yet every relation admits of two standpoints, just as the line _AB_, which may serve to represent a certain and definite relation, is determinable from both ends, _A_ as well as _B_. Let us call _A_ the subject and _B_ the object. Neither _A_ nor _B_ is a reality, a whole complete _Wirklichkeit_. A thing in order to be real must be active, it must work, it must stand in relation to something else. _A_ is a mere mathematical point, but _AB_ representing a process does something, it performs work, it is real. A thing in itself, if it could exist at all, would be tantamount to non-existence, it would represent a _Sein_ without being _Wirklichkeit_. When bearing this in mind, it appears natural that the oneness of existence, representable in such relations as is that of _AB_ = -_BA_ will admit of two standpoints, _BA_ representing subjectivity, and _AB_ representing objectivity. We can consider the relation of the world at large to one special point (which latter may in its turn stand for a whole system of relations) or vice versa the relation of this point to the world at large. The former standpoint is that of the microcosm, or the soul, the latter that of the macrocosm or the universe; the former results in awareness, the latter appears as matter in motion. The former is subjectivity, the latter objectivity.