Selection from – Ethics – Part I. Of Deus sive Natura (Page 8)
Spinoza's Words: (on the necessary nature of things)
Proposition. XXXIII. Things could not have been brought into being by (Deus sive Natura) in any manner or in any order different from that which has in fact obtained.
Proof—All things necessarily follow from the nature of (Deus sive Natura). [They] are conditioned to exist and act in a particular way. If things, therefore, could have been conditioned to act in a different way, so that the order of nature would have been different, the nature of (Deus sive Natura) would also have been able to be different; and therefore that different nature also would have perforce existed, and consequently there would have been able to be two or more Gods. This is absurd. Therefore things could not have been brought into being by (Deus sive Natura) in any other manner, &c. Q.E.D.
Comment:
Nature being what it is produces things as they are. If, says Spinoza, things could be different they would require a different Nature as cause – making for two different Natures, which he deems impossible. Therefore, in Spinoza's philosophy, things produced by Nature as they are; neither good or bad, nor could they be different. Therefore, it is implied, they must be accepted as Nature gives them to us.
Many have interpreted from the above proposition that Spinoza was a determinist and have dismissed him as such. But this is an error. Spinoza is a "necessarialist." That is he sees that there is a necessary aspect of basic things. (Deus sive Natura) cannot cause something at one moment in time and then later cause the reverse. There is order within the Laws of Nature. They do not vary. The things the Laws produce are necessary not in the sense that they are needed but in the sense that they are unavoidable.
In the everyday world around us most things are contingent – that is, they don't have to be the way we find them, they are merely possible and likely to change or be changed or they could have been different. Therefore, we tend to see everything as having the freedom to change or be different. That may be an illusion. Limited as we are, we may not be capable of knowing the long chains of cause and effect of things and actions.