The Works of Benedictus de Spinoza

...common source, must be said and called. But indeed, all this unity of the subject with the object is not necessarily so, except when the subject thinks. Furthermore, all deduction of derived principles or special reasons from one absolute and inherently true principle does not occur except in the realm, so to speak, of reason or formal truth. Finally, that necessary identification of different things, without which there would be neither affirmation nor negation, also pertains to the form of thinking. But whether two objects, which are present in the mind and are currently taken by the given thinker as one for a certain reason—that is, are conceived under the form of unity—are actually one by their own nature, which we are compelled to attribute to things that we cannot help but think of with the notion of existence, or whether they merely seem to the thinker at this moment to be one, that thinking subject, no matter how much he himself becomes a subject-object (that is, thinks from his subjective unity with the object), cannot effect or decide this while judging formally. This, of course, depends on the matter; everyone proclaims or senses this. But matter (I imply the internal matter of thinking) does not depend on that manner by which it is now thought by this or that person, that is, reduced from diversity to unity. Rather, the ideal object—that is, that which is in the mind and because of which the mind can rightly be called a subject-object—whenever it confronts the mind to be thought of with existence, is subject to necessity or the reality of its own nature, such that an ideal object is identified with another formally in vain and erroneously, unless it must be identified necessarily and through...