Few people in the history of philosophy have the theoretical depth and persuasive power of Spinoza. Born in Amsterdam in 1632 into a family of Spanish Jews who emigrated first to Portugal and then to Holland, he was not only a philosopher of fundamental importance but also an expert lens-maker by trade. His biography cannot be adequately described here, however, it is sufficient to mention two essential episodes in his life. In July 1656, he was excommunicated for his heterodoxy by the rabbis of Amsterdam and forbidden to reside in his hometown, which forced him to move to Ouverkerk. He then had to move again from Voorburg to The Hague to escape the controversy surrounding his Theological-Political Treatise (1670), despite being published anonymously, and in 1677 died in The Hague.
As already mentioned previously, Descartes had adopted the Anselmian ontological argument (the supremely perfect Ens can only compete with existence, otherwise if deprived of it, it would no longer be ‘supremely perfect’), essentially to demonstrate God’s existence. Spinoza in turn takes up Descartes’ argument, and it soon becomes clear that this Ens cannot have the properties attributed to it by Christian theology – thus, a fortiori, by Jewish theology. In his masterpiece – the Ethica more geometric demonstrata – Spinoza starts from the definition of substance, and from it proceeds through propositions, scolia, demonstrations and corollaries, the fundamental structure of his ontology. But let us proceed in order.
He is the cause of himself, he is infinite, and he eternally produces himself.
Spinoza’s God is Being itself.
Spinoza starts from Descartes’ hyperbolic doubt and considers it necessary to give a different answer from the one proposed by Descartes himself. Therefore, in doubting the existence of everything, including ourselves, what can we be certain necessarily exists? Spinoza’s answer is substance. And this substance is precisely the starting point of the first part of the Ethica (which deals with God), and presents some ‘definitions’ at the beginning:
Spinoza defines substance as that which needs nothing other than itself in order to be and in order to be understood. It requires nothing to justify its existence, nor any other concept that might approach it in order to determine it. Substance is what it eminently is, and it needs nothing else than itself to be, to exist. It is therefore clear that by the I definition, substance is causa sui, that is to say, the cause of itself. This concept of self-production, of being the cause of oneself, appears here for the first time in the history of philosophy. Previously we had the concepts of efficient or final cause (from simple causality to the unmovable motor to creatio ex nihilo) and of eternity (that which has no cause has existed forever and forever will). Here, however, there is something different: substance has total and absolute logical and ontological independence. In this way, Spinozian substance also escapes the possibility of falling under Descartes’ hyperbolic doubt: it is independent and produces its own existence and knowability.
In the II definition (omitted above), Spinoza also specifies what he means by ‘finite thing’, i.e. by ‘finiteness’. He wrote that a finite thing is such because it is limited by another thing of the same kind (which is also finite). One thing is self-evident: a finite entity can only be accompanied by another finite entity, for if either of the two were infinite it would either have to incorporate the other or it would not be infinite, because it would be limited by the very existence of the other. In other words, if the infinite is indeed infinite, then it is boundless throughout. However, if there is a finite place in which it is not, then it is no longer infinite.
This thought allows us to determine the following: if the only thing whose existence we can undoubtedly recognise is substance (inasmuch causa sui) and it needs nothing else to exist, then substance is the only thing that exists, and is therefore infinite.
Let us continue with Ethica:
Substance exists, and it is infinite, but in order to understand it, it must be determined in some way. If we take the human intellect as an example, it is able to grasp two attributes of substance: res cogitans and res extensa; the first is thought, the second is physical extension. Yet an infinite substance possesses infinite attributes, and each of them is infinite. But the things that we perceive and that surround us are finite entities – for example, thoughts and everyday objects. So they are not attributes of the substance, but manners, affections of the substance that exist because of something else (none of our thoughts or the things we experience are causes of themselves, but are all caused by something else). This is how Spinoza’s ontology is structured: there is substance, which is the cause of itself and is infinite, that is, it saturates the entire field of being; from it proceed infinite attributes, each of which is in itself equally infinite. From the infinite attributes proceed the manners, or affections, which are finite. For the human intellect, the only two perceptible attributes are thought and extension (matter), from which the finite entities we experience in our lives (thoughts and objects) proceed ‒ the human intellect itself is a manner of the attribute res cogitans of substance.
Having said this, we can begin to understand the VI and final definition:
The substance that has been spoken of so far is therefore, for Spinoza, God. He is the cause of himself, he is infinite, and he eternally produces himself. Attributes and manners are nothing but a manifestation of God, that is, part of his essence. Even human beings, who are made of mind and matter, are manners of God’s substance. The reason for the excommunication should be clear at this point. Spinoza’s God is Being itself. This God is not free to create, if not accordingly to his own nature and nothing can prevent him from doing so. And his creation is not external but within himself. Take anything you have ever experienced: every human being you have ever met, every object you have ever picked up, every tree that has sheltered you from the sun, every blade of grass you have ever plucked, every grain of sand you have ever let slip through your fingers, every ‘fluff bunny’ you ever swept away; and every idea, every thought, every sorrow, every joy, every sorrow, every joy; all this and much more, all this and everything else is God.
God is the immanent cause of reality, so the world has not been created. It is constantly generated by God in God: as generating power is Natura naturans, insofar as generated reality is Natura naturata. Hence, the well-known expression: Deus sive natura (God or nature), where nature is understood in the broadest possible sense of the term, it is to some extent the physis of earlier Greek philosophers. It is unusual that because of this expression the idea of an atheist Spinoza has been handed down, when it is quite the opposite: how can one, who believes that only God exists in the fullest and deepest sense, be an atheist?
There would be many other things to clarify, some of great significance and others of more specific theoretical interest (for example: how does one go from infinite attributes to finite manners? How does the well-known Spinozian formula “Omnis determinatio est negatio [Every determination is a negation]” fit into all this?), yet it is not possible to go into further detail here. Benedict de Spinoza’s thought is so profound and vital that it would take more than one article to begin to restore it in its worthy extent.
 Benedict de Spinoza, Ethics [Ethica more geometrico demonstrata], in Ethics preceded by On the improvement of the understanding, edited with an Introduction by James Gutmann, Hafner Publishing Company, New York 1954, p. 42.