Throughout the history of metaphysics, the question: What is? It has always been answered in an incomplete, unsatisfactory or complicated way, but Spinoza attempted to answer this question in an exceptional way by simply describing God and his essence. According to Spinoza's views, the qualities of God can be considered attributes and the modes are only affections of a substance. This article will provide a detailed view of Spinoza's key ontological definition of God as a unique substance, its attributes and their correlations. The study goes further by exploring the main scientific argument between Spinoza and Descartes, concerning their view of substance and its attributes. Descartes and Spinoza seem to have different perceptions regarding the existence of substance. However, the two researchers have comparable perceptions in some respects. They both refer to God as the primary substance. One thing that Spinoza and Descartes seem to agree on in general is the definition of substance. According to Spinoza, a substance is nothing other than a thing which subsists in such a way that it does not depend on any other thing for its survival. In the introduction to his work, The Ethics, Spinoza illustrates substance as “what it is conceived by itself and in itself.” He explained this to mean that a substance does not need any feeling of anything else to exist, which also seems to coincide with Aristotle's interpretations of how a substance exists, according to which it is independent of all other things. (1).The fundamental trait of substance, as Spinoza expresses it, is its independence. Spinoza defines God as a totally unlimited substance, or a substance "comprising an infinity of attributes", each of which illustrates a...... middle of paper ...... FS 1953, Spinoza Definition of l 'attribute, Philosophical Review, vol. 62, no. 4, pp. 499-513.Kline, G 1977, On the infinity of Spinoza's attributes, London: Routledge.Melamed, Y 2012.The constituent elements of Spinoza's metaphysics: substance, attributes and modes (08.14.11). Johns Hopkins University. Available at: http://philosophy.jhu.edu/wp-content/uploads/sites/7/2013/08/Spinozas-Metaphysical-BuilBlocksFinal.pdf [Accessed December 5-15, 2013] Nadler, S 2006, L' Spinoza's Ethics: An Introduction, Cambridge: Cambridge University Press. Pollock, F 1966, Spinoza: His Life and Thought, New York: American Scholar Publication. Rocca, MC 2008, Spinoza: The Routledge Philosopher Series, London: Routledge.Scruton, R 1999, Spinoza. London: Orion. Woolhouse, RS 1993, Descartes, Spinoza, Leibniz: the concept of substance in 17th century metaphysics, London: Routledge.