The Works of Benedictus de Spinoza

...the reality of its own nature. Whether this happens or not, however, is the very knot, the very reef, which cannot be removed or moved from the middle by means of a conceived formal unity and that ideal identity of the subject with the object.

But it is nevertheless very prone for people to take the unity of the ideal object with the subject, which truly exists while you are thinking, for a real unity that is also necessary outside the form of thinking. It is further prone that those especially who are occupied primarily with thinking—namely, those who are philosophizing—should apply that reduction of diverse things to unity, which is necessary in the act of thinking or enthymēsei meditation, and the elevation of special reasons and laws to one principal reason (which they call the supreme principle), even to things insofar as they are compelled to conceive of them as existing, that is, insofar as something is attributed to them by the very act of thought by which they are posited as being outside of thought. If anyone makes this application fully and purely, he will reduce the to pan the all in existing to to hen the one. He has become accustomed, namely, in thinking, to always consider two different things as one through a certain intuition; and thus, while he thinks, he proceeds to infinity. Now, therefore, he transfers (as indeed very similar things can be confused even by the most sagacious for some time) the formal method of thinking beyond its own bounds, that is, to the connection of existing things. He takes the law of thinking for the law of existing. Once this is obtained or granted, no system could ever be conceived as more true or more consistent with itself than that of Spinoza.