We said at the outset: The criterion of truth includes the criterion of reason. Truth, like reason, consists in developing a general concept, or an abstract theory, from a given sum of sense perceptions. Therefore it is not abstract truth which is the criterion of true understanding, but we rather refer to that understanding as being true which produces the truth, or the general hall-mark of any concrete object. Truth must be objective, that is to say it must be the truth about some concrete object. Perceptions cannot be true to themselves, they are true only in relation to some definite object, and to some outside facts. The work of understanding consists in the abstraction of the general hall-mark from concrete objects. The concrete is the measure of the general, the standard of truth. Whatever is, is true, no matter how much or how little true it may be. Once we have found existence, its general nature follows as truth itself. The difference between that which is more or less general, between being and seeming, between truth and error, is limited to definite conditions, for it presupposes the relation to some special object. Whether a perception is true or false will, therefore, depend not so much on perception as on the scope of the question which perception tries to solve of its own accord or which it is called upon to solve by external circumstances. A perfect understanding is possible only within definite limits. A perfect truth is one which is always aware of its imperfection. For instance, it is perfectly true that all bodies have weight only because the concept of "body" has previously been limited to things which have weight. After reason has assigned the conception of "body in general" to things of various weights, it is no longer a matter for surprise to find that bodies must inevitably have weight. Once it is assumed that the term "bird" was abstracted exclusively from flying animals, we may be sure that all birds fly, whether they are in heaven, on earth, or in any other place. And to explain this we do not require the belief in _a priori_ conceptions which are supposed to differ from empirical conceptions by their strict necessity and generality. Truths are valid only under certain conditions, and under certain conditions errors may be true. It is a true perception that the sun is shining, provided we understand that the sky is not covered by clouds. And it is no less true that a straight stick becomes crooked in flowing water, provided we understand that this truth is an optical one. _Truth is that which is common or general to our reasoning faculty within a given circle of sense perceptions. To call within a definite circle of sense perceptions that which is exceptional or special the rule or the general, is error._ Error, the opposite of truth, arises when the faculty of thought, or consciousness, inadvertently or shortsightedly and without previous experience concedes