Spinoza (1632-1677) was profoundly knowledgeable about the ideas and discoveries of Kepler, Copernicus, Galileo, Hobbes, and Descartes. He attempted to use their thoughts and discoveries to combine the scientific and philosophical thinking of his time.
Spinoza was troubled by Descartes’ philosophy because Descartes doubted everything until he found premises which could not be doubted. As a result, he ended up with mental substances he was sure of, because he could not doubt the existence of his own thinking mind. And Descartes also concluded that substances must exist outside his mind, or he could not have mental images of them in the first place.
This alleged chasm between mental and physical substances disturbed Spinoza. He reasoned that if Descartes was correct, there was no way a mind could interact with physical objects or simply put, to move them about in space.
Spinoza’s solution is beautifully simple. Since God is infinite, he can have no boundaries. God must be co-equivalent with everything (The Story of Philosophy). Thus, if we describe planets, stars, water, people, all mental ideas of substances, all actual substances in the real world, we are merely describing the same reality—God.
To Spinoza, the being of God is ordered by absolute truth in much the same way that mathematics and physics are ordered. Ultimately, these logical systems follow definite laws which cannot be otherwise. Even when laws are found in error via scientific experimentation, each time their paradigm changes we are carried closer and closer to absolute truth.
God, then, is not outside the world any more than substances are inside or outside the mind. Nor is he inside anything that exists. God is everything that exists as one single unit. Spinoza's theological pantheism is beautiful in its simplicity.