If, as Spinoza argues, there is only one substance – God – which is infinite, then there can be nothing outside or separate from this God. Precisely because God is a limitless, boundless totality, he must be an outsideless whole, and therefore everything else that exists must be within God. Of course, these finite beings can be distinguished from God, and also from one another – just as we can distinguish between a tree and its green colour, and between the colour green and the colour blue. But we are not dealing here with the distinction between separate substances that can be conceived to exist independently from one another.
Again, this is rather abstract. As Aristotle suggested, we cannot think without images, and I find it helpful to use the image of the sea to grasp Spinoza's metaphysics. The ocean stands for God, the sole substance, and individual beings are like waves – which are modes of the sea. Each wave has its own shape that it holds for a certain time, but the wave is not separate from the sea and cannot be conceived to exist independently of it. Of course, this is only a metaphor; unlike an infinite God, an ocean has boundaries, and moreover the image of the sea represents God only in the attribute of extension. But maybe we can also imagine the mind of God – that is to say, the infinite totality of thinking – as like the sea, and the thoughts of finite beings as like waves that arise and then pass away.
Spinoza's world view brings to the fore two features of life: dependence and connectedness. Each wave is dependent on the sea, and because it is part of the sea it is connected to every other wave. The movements of one wave will influence all the rest. Likewise, each being is dependent on God, and as a part of God it is connected to every other being. As we move about and act in the world, we affect others, and we are in turn affected by everything we come into contact with. . . .