Spinoza is undoubtedly one of the greatest rationalist philosophers of the West. He developed the Cartesian theory of Substance into a full-fledged system of symmetry and perfection. To Spinoza there is only one Substance, God, and this he accepted in agreement with one of the aspects of the philosophy of Descartes. All things in the world follow for Spinoza from the supreme Substance, not as evolutes of it in the process of time, but in the manner of corollaries of a geometrical theorem. The universe is necessarily deduced from the one Substance as we deduce mathematical truths in our calculations and reasonings. Space, time and objects are all modes of the one Substance. Spinoza does not give time a separate reality; to him there is only eternity and time is only a mode of thought. Anticipating Hegel, as it were, he argues that the conclusions arrived at logically are not different from what exist really. He would agree with Hegel that logic and metaphysics are essentially one. To Spinoza thought and reality lose their distinctness and become one. Spinoza conceives the universe as an interrelated system in which every element is accommodated as an indispensable and necessary feature in the exact place assigned to it. The universe is a strictly determined whole and becomes rigid with the absence of any purpose or final aim directing it beyond itself. Spinoza makes thought and extension, the properties of the mind and matter in the philosophy of Descartes, the two attributes of the absolute Substance, and thus a greater consistency and method is seen in the system of Spinoza than in that of Descartes.

Substance is God, and, being independent, it is also infinite. All finite things are dependent on some other things. The Substance is its own determination, nothing else can determine it; it is not dependent on anything else. The great motto of Spinoza is that all determination is negation, and so the Substance is free from the determination of individuality or discreteness. God, being infinite, cannot be possessed of the psychological organs or be endowed with the volitional and intellectual functions known to man, which are valid only on a dualistic basis. Spinoza differs from Descartes in his view that God and the world are not two distinct principles. He merges God in the world and the world in God. Thus we get a pantheism where God is the world and the world is God. Students of Spinoza have, however, endeavoured to discover a transcendent aspect of the Supreme Substance and save him from the charge of pantheism.

Thought and extension are considered by Spinoza to be the two outstanding attributes of the supreme Substance, God. God has infinite attributes, but out of these only thought and extension are intelligible to man. These two attributes are everywhere, for they are inherent in the Substance which is infinite. There is no part of the Substance which is not defined by thought and extension. Spinoza is inclined to make each of these attributes infinite in nature, though on account of his endowing God with infinite attributes he is hesitant to make them absolutely infinite. The theory of parallelism which Descartes propounded finds a place again in Spinoza's system, though in a modified way. Spinoza holds that thought and extension cannot have interaction between themselves, for they are the inward and outward expression of one and the same process. One and the same entity appears as mind within and matter without. The order and connection of mental phenomena is not dissimilar to that of physical phenomena. The two laws run parallel to each other in their method and working. Mind and body are consequently considered to be modes of one process, having one law, and, thus, they cannot exercise influence on each other in any way. Thought and extension have equal reality and are subsistent in the infinite Substance and proceed from it as necessarily as mathematical deductions. There is no substance independent of God, Who is the supreme Substance and Whose attributes are thought and extension. In short, God, to Spinoza, is a thinking and extended being, which would mean that God is possessed of mind and body, though by God's mind and body Spinoza does not mean the mind and the body with which we are familiar, but the mental processes scattered over all space and time and the physical processes that constitute the stuff of the world. While Spinoza dismisses the dualism of substances admitted by Descartes, he accepts the same by making them attributes of the supreme Substance. The same difficulty remains, though the terminology in which it is expressed is different, and the rigour of the dualism is attempted to be overcome by its association with the One Substance.

Spinoza holds that Nature is in reality the one universal Substance, and its appearance as consisting of diversified phenomena is the result of our imperfect ways of looking at it. Everything in the world is an attribute or a mode of the eternal Substance, and its existence is the reality of all things. Spinoza goes beyond Descartes when he thinks that God and mind, too, are determined by the laws of mechanics. Spinoza makes strict determinism prevail in Nature. Purpose and design are to him delusions transferred to the objective universe by the limited vision of individuals. The will of God and the laws of Nature are not two different things, but mean the same thing. The laws are unchangeable and mechanical. There is a distinction, however, made by Spinoza between his conception of the supreme Substance and the ordinary view of substantiality or concreteness which many are likely to hold in regard to substance. By Substance Spinoza means essence or ultimate existence and not corporeal matter. He identifies his Substance, or God, which is the cause or origin, with what he terms Natura Naturans, as distinguished from the visible physical universe of diversified bodies, which is merely an effect and which he calls Natura Naturata. Spinoza's God has no will or intellect of the ordinary kind. He identifies God's Will with the totality of all causes and laws and God's Intellect with the totality of all minds in the universe. Thus, it appears that his God is in all ways the sum-total of individualities.

In the philosophy of the Vedanta, time is not a mode of any individual's mind but is necessarily valid to all minds. It is a part of ishvara-srishti and it can be called a mode of thought only when this thought is identified with the cosmic Will of Ishvara. All individuals are in time and no one creates time. Space and time are the necessary presuppositions of all perceptions. Even the ideas that arise in the mind of man are determined by the properties of space and time. Sensation, thinking, understanding and reasoning are all dependent on the universal properties of space and time. It is true that there is only eternity, and time is a relative appearance, but it has to be added here that this appearance is not the product of any individual's thought, but is the determining factor of all individual thoughts. Time belongs to the cosmos and hence it is an extramental reality. The Vedanta would agree with Spinoza that time is a mode of thought only when this thought is identified with God's Thought.

Spinoza's view that the universe is determined and rigid without any purpose or design directing it is not fully acceptable. The Vedanta makes a distinction between the universe as such which it calls ishvara-srishti, and the universe in relation to the individual which guides the processes of a secondary universe, which it calls jiva-srishti. When it takes into consideration the universe as such, the Vedanta would agree with Spinoza that it is determined and has no purpose beyond itself. For, the universe as it is in itself, independent of individual perceivers, is the body of Ishvara, and it is its own end. It has no other aim which may determine or direct its processes. God's Will is an eternal law, without a beginning and an end, and, as the universe as such is the very body of Ishvara, it must be eternally determined in its workings, allowing in no change, modification or amendment of any kind. Cosmic determination, relentless and immutable, is the law of the universe of Ishvara. But in the relative universe, which is what is observed by the individuals, there is purpose, design, aim, an ultimate goal. We cannot deny the fact of change in this universe. Change is movement and movement cannot be merely a chaotic changing of positions without a directing principle behind it. All change is movement towards an aim, a fulfilment in a higher principle, which is more inclusive and which transcends all the lower ones. The realisation of the highest perfection in the consciousness of what does not admit of any further transcendence is the ultimate directing principle of all movements seen in the world and the individuals. In other words, God-realisation or Self-realisation is the goal of life. Thus, there is a purpose in the workings of Nature, of which the different individuals are parts and which constitutes their environment with which they are inextricably bound.