Proposition 7 of Spinoza's Ethics book 1 says that 'it pertains to the nature of substance to exist'. This is proven by using (the corollary to) Proposition 6 to say that nothing can produce substance. From which one then concludes that substance much cause itself, end hence existence is part of its essence.
My issue is that i don't we see why it follows that a substance much cause itself, simply because it cannot be produced. Assuming the principle of sufficient reason (which i guess Spinoza does) I can see that this holds if we assume additionally that the substance under consideration exists, because then the substance must have a cause. So perhaps Proposition 7 should read 'If a substance exists, then it pertains to the nature of that substance that it exists'. But if one adds this extra assumption of existence then one can no longer use Proposition 7 to prove the existence of 'god' in Proposition 11.
Any insights on where i am going wrong are very welcome.