I want to continue by taking up the treatment of substance and God and their role in the epistemology we’ve been discussing. First, let's return to Spinoza's account of his method. In some ways it is rather like Descartes's account, but importantly different in that right at the start we find that an idea which causes us to doubt is not clear and distinct. Spinoza's point is that doubt is by nature a state of confusion. He goes on to say that sometimes the deceiver may incline us to doubt things represented to us – but the doubt is again removed. And you will notice there are very considerable differences from Descartes. Spinoza is quite clear that doubt we may have of the deliverances of the senses is corrected by more careful use of observation. He doesn't make Descartes's move of trying to cast doubt on the deliverance of the senses as a whole:

In other words, until we reflect on the knowledge we have of the first principle of all things and find that knowledge teaches us that God is not a deceiver – and we know this when we know God with equal certainty [as] that a triangle's angles are equal to two right angles. And if we have knowledge of God equal to that of a triangle, doubt is removed.

This sounds, if one doesn't read it carefully, as if Spinoza is envisaging an argument like Descartes's – doubt is brought to an end by a truthful God who removes the doubt – but not in the same way. Whereas in Descartes, philosophical doubt has already proceeded some way and one is relying on doubt as the method of ensuring ideas are clear and distinct, in Spinoza the recognition that God is the first principle of all things and is not deceitful is not, as in Descartes, an external guarantee which shows we needn't doubt anymore – on the contrary, there is a recognition that doubt of that sort is impossible: to try to doubt the reliability of thought is just confused.