Spinoza was a philosopher's philosopher. Bertrand Russell said of him that he was 'the noblest and most lovable of the great philosophers'. Unfortunately this did not prevent him from being vilified, excommunicated from his Jewish community, and labeled a heretic during his time.
Spinoza was among the most important of the post-Cartesian philosophers who made significant contributions in every area of philosophy. Along with Descartes and Leibniz, he is known as one of the three major Rationalists.
He is best known for his work The Ethics, one of the classics of modern philosophy (published after his lifetime). In it he sets out his metaphysical ideas, which begin with the notion that reality comprises of just one substance, which can be conceived of as either Nature or God. This substance has infinitely many attributes, however, we as finite human beings can only perceive two of them, extension and thought.
Unlike Descartes, who put forth that mind and body are two separate types of things, Spinoza argued that mind and body are merely different ways of conceiving the same reality. Starting from basic assumptions and by a series of geometric proofs (along with many other philosophers, Spinoza believed that mathematics was the means to discovering the truth about the universe) he constructed a universe that was also God. This is the classic example of pantheism - the belief that God and the universe are one and the same. Since God and Nature are one, God is immanent (not transcendent), self-creating and entirely free.
As far as individuals go, according to Spinoza, each individual is a localized concentration of the attributes of reality and the way to liberty is by the means of one's intellectual powers.
As the supreme rationalist, Spinoza held that there are three levels of knowledge. The lowest level of knowledge or 'vague experience' is acquired through the senses. For Spinoza and the other Rationalists this is not knowledge at all and real knowledge is always the conclusion of deductive reasoning. 'Adequate ideas' or common notions provide the second level of knowledge and form the basis for the third level of knowledge. An adequate idea is one that is logically coherent and the test of its truth is that logical coherence.
Spinoza's highest level of knowledge is 'intuitive knowledge' which uses adequate ideas to know the 'essence of things'. This knowledge is the intellectual love of God because it comprehends everything in relation to God and recognizes God as the source and connection of all things. As finite human beings, however, we have only a limited understanding of such things. We must therefore actively seek knowledge because the more objects the mind understands by the second and third kinds of knowledge, the less it suffers from harmful emotions and the fear of death.
Spinoza's philosophy was ahead of his time, in that, today our planet is viewed as a single vast organism or self-regulating cell. His system also suggested a holist ethics similar to that put forth by modern ecologists. It implied that if you harm the world, you harm God, if you harm others, you harm yourself.