Principle of Sufficient Reason
[1] Spinoza deduces many things from his concept of G-D, but one in particilar deserves mention for its central role in subsequent controversies. In Spinoza’s world, everything that happens, happens necessarily. One of the most notorious propositions of the Ethics is 1P3: “Things could not have been produced by G-D in any manner or in any order different from that which in fact exists”. This is a logical inference from the proposition that the relation of G-D to the world is something like that of an essence to its properties {circle to roundness}: G-D cannot one day decide to do things differently any more than a circle can choose not to be round, or a mountain can forswear the valley that forms on its side. The view that there is a “necessary” aspect of things may be referred to by the sometimes inappropriate name of “determinism:”
[2] Of course, Spinoza acknowledges, in the world we see around us, many things seem to be contingent—or merely possible, and not necessary. That is, it seems that things don’t have to be the way that they are: the earth might never have formed; this book might never have been published; and so on. In fact, Spinoza goes on to say, every particular thing in the world is contingent when considered solely with respect to its own nature. In technical terms, he says that “existence” pertains to the essence of nothing—save G-D. Thus, at some level, Spinoza stands for the opposite of the usual caricature of the determinist as reductivist, for, according to his line of thinking, we humans are never in a position to understand the complete and specific chain of causality that gives any individual thing its necessary character; consequently, we will never be in a position to reduce all phenomena to a finite set of intelligible causes, and all things must always appear to us to be at some level radically free. (In this sense, incidentally, he should count as a radical empiricist.) In somewhat less technical terms, we could say that, from a human point of view, everything must always seem contingent; even though from a divine or philosophical point of view, everything is nonetheless necessary. From the philosophical point of view—and only from the philosophical point of view—the distinction between possibility and actuality vanishes: if something may be, it is; if it may not be, it is not.
—Matthew Stewart’s The Courier and the Heretic; 2006