Substance is in abstraction a univocal core concept in Western philosophy. Univocal in the sense that under substance is understood an 'existence' or 'being'. This univocality loses itself however in a diversity of conceptions when substance is going to be considered more concretely, such as among other things in God and man. Every philosopher here seems to handle his own conceptions and definitions, and the Western philosophical tradition has with that left us a rich palate of ideas and concepts. Descartes and Spinoza are two famous philosophers with such a legacy. They too have had their own thoughts on substance in relation to God and man, and came each to their own conclusions and definitions. Let us consider both their conceptions closer and also compare them to each other.
A consideration on Descartes' ideas on substance, God and Man can be difficultly placed loose from that which brought him to start to handling these ideas. It is the context of Descartes' causes that make his ideas more understandable. The context to which is referred here regards Descartes' life and his famous meditations.
René Descartes was born in the neighbourhood of Tours on 31st of March in the year 1596 and was schooled by the Jesuits that imparted to him a Catholic image of God. Despite his later philosophical ideas would Descartes keep this Catholic image of God for the rest of his life.1 The rest of his life but with an interlude that took place during his meditations. On the 10th of November in the winter of 1619 he came to his method of doubt which he would later work out in the 'Discours de la Methode'.2 The following of the four steps of this method of doubt3 brought him in first instance to a radical turnaround regarding his image of God, namely to a doubt of the existence of a good God. Firstly there was the doubt about the reliability of the senses. The assumption that raised this doubt in him, namely that he surely knew whether he was waking or sleeping, was parried by doubting a difference between waking and dreaming. This doubt brought him thereafter to the assumption that however, dreaming or waking, mathematical truth could not be doubtable. Or could they? Maybe he was in the power of an evil demon, instead of a good God, that made him think that three plus two was five, while in reality this perhaps was six.4 With this doubt disappeared for the moment the assumption of the existence of a good God as being obvious.
Very soon however would that God for Descartes rise again out of the ashes of His absence. For doubting Descartes stumbled eventually upon a by him presumed fundamental surety in which no demon could trick him, namely his own existence, by himself formulated in the famous words: "I think therefore I am."5 From this fundament would Descartes eventually give God and our surroundings again value of reality. Two God proofs were postulated on base of the 'cogito, ergo sum'; an idea-theoretical proof of God and an ontological proof of God. The idea-theoretical proof of God of Descartes starts with a proposition and two assumptions. The proposition regards the statement that every cause of an idea has at least as much formal reality as the idea has objective reality.6 The two assumptions that Descartes has in hands regard himself as ending substance and the idea of God, the endless and perfect substance. Now leaving from the proposition that every cause of an idea has at least as much formal reality as that the idea has objective reality it cannot be so that an ending substance is the cause of an idea of an endless substance. Only a true existing endless substance can be the cause of such an idea, only God can be the cause of the idea 'God'.7 From this proof of God then Descartes is able to come into a new trust regarding the reality of the empirical world and physics. For God is perfect, and deceit is an imperfectness that does not suit a perfect God.
Although above the veil has been lifted a little already is an important question in this consideration how Descartes sees and defines God. And how does that God relate to Descartes' conceptions regarding substance and man? Descartes means with the name of God: "an endless substance, eternal, unmovable, independent, all knowing, all powerful, and by which I myself, and all other things that exist, if there are such, are created."8 The relation between God and substance is thus clear: God is an endless substance. The relation on itself between God and man is also clear: man is created by God. But how does Descartes see that created man itself?
The consideration of man as created by God places man in the position of an on itself standing substance. Nevertheless does Descartes' philosophy entail a lot more regarding conceptions on man. For it is from considering his experiment of doubt that he introduces a certain duality in man. Namely, such was Descartes' flow of thoughts, if there can be doubt about the existence of the own body but not about the existence of the own mind, then body and mind must be different substances. So man is according to Descartes a dualistic being consisting of two substances: body (extendedness) and soul (mind).9
With this Descartes established a right for existence of science and theology. Science could occupy itself with research on the bodily level, and theology could remain its authority on statements regarding the soul.10 An important question that received however no answer from Descartes was how the interaction between body and mind in such a dualistic concept can be explained.11
More than a century after Spinoza's death the German poet Novalis called him "the God-intoxicated man." But in his own time and long after he was mocked as being to the bone atheistic.12 That one and the same philosophy can be taken by different beholders in almost contradictory terms is a remarkable given. A first question that this remarkable given gives rise to is logically how Spinoza himself thought about God. In this work 'Ethica' he pithily mentions several meaningful essential qualities of God.13
In these givens is actually Spinoza's total frame of concepts of God, substance and man contained. An elucidation with back-reference shall make this clear.
Substance according to Spinoza is not different from God. Both are one and the same. For substance exists necessarily and exists only from its own necessary nature. These qualities we recover at the, by Spinoza to God given, essential qualities (see a and c). Also can such a definition according to him be nothing less than the totality (or the cause of the totality) of all that is, was and shall be. There can, according to Spinoza, exist no more than one substance, and that substance is endless and is God.14
The existence of the diversity of the us surrounding world Spinoza explains through 'modi'. Modi are modifications of God or of the endless substance. As this goes for the us surrounding objects, so this also goes for man. People are not individual substances but modifications of God. A God that is not separate from us as creator but in which we are contained, are part of, and that in this way still is our cause.15 (See e, f and g).
With this Spinoza however does create a difference between two principles of that one God. At one side God in His unity as eternal and only substance, on the other side as a totality of all modi. Spinoza names these the 'natura naturans' and the 'natura naturata'. The natura naturans is God considered as a free cause and determined by nothing but Himself. The natura naturata is God considered in His diversity of modi without which he cannot be understood.16
When we consider both philosophies then we see that they start with total different points of departure. Descartes finds his first evidence through his method of doubt: namely his own mind and the for him thereto related 'being'. He seeks the first evidence as a firm fundament in himself to build then thereon the rest of his philosophy. Spinoza in contrary seeks the first evidence not within himself but postulates an own definition of God as first evidence. His entire philosophy seems further contained in that pithy definition, at least where it regards substance, God and man. Now Spinoza's contentual definition may be admirable because it carries within itself so much plausibility, Descartes' first evidence is much more argumentative than that of Spinoza. Spinoza does not clearly indicate how he has come to his evidence, where Descartes elaborately deals with this.
Where God is concerned it is clear that Descartes' conceptions of God are definitely not those of Spinoza. Descartes seems in his image of God to stay loyal to the anthropomorphic Catholic idea of transcendental God that stands loose from His creation. Spinoza however wages fiercely battle against such a conception17 and defines God as an only and all-encompassing substance that is immanent (or perhaps here the word 'inherent' is even better in place) to the world and the universe. Descartes' idea-theoretical proof of God, it is said, is absolutely inventive. However it stands or falls with the assumptions that every cause of an idea has at least as much formal reality as that the idea has objective reality, and that man is an ending being and that God as endless reality also has a reality value. These assumptions are however not uncontroversially acceptable as true.
Where Descartes regarding God and creation already takes a dualistic point of view he does this again when he considers man. For man he sees as being two different substances: the 'res extensa' (extendedness) and the 'res cogitans' (mind). Spinoza holds fast to a per definition monistic image. Man is only a modification of that one substance. The difficulty with the postulating of a dualistic image of man such as that of Descartes is related to the question how the interaction between both substances of body and mind is explainable. The discussion about this stays to the present day still indecisive.
The problem of the philosophy of Spinoza regarding substance, God and man is that the philosophy is difficult to verify and at the same time difficult to falsify. His ideas seem captured in one all-encompassing assumption, namely his definition of God. Within that assumption do all ideas (substance, man, God) get a plausible place. There is however no different argumentation found for handling the definition than that which we find within the definition itself. This invites to critical examination of the definition on meta levels before stepping inside Spinoza's synthesized image of the world.
Descartes' philosophy is much more argumentative than that of Spinoza, although his arguments are certainly not always unquestionable. The largest suspicion in Descartes' philosophy is however the following: Through his experiment of doubt he thought to cast out all his presumptions to after that start anew from an absolute evidence. How suspicious does such an experiment become when he stumbles upon conclusions that acknowledge his presumptions of before the experiment; namely a Catholic image in which all Catholic elements of body, soul and creator are present, and in which also is place for his passion; science. This needs to say nothing about Descartes' sincerity, but a critical consideration of his experiment of doubt seems here in place. A critical consideration which however not necessarily needs to say anything about any other experiment of doubt.