Full Idea
Spinoza could only think his philosophy, not believe it, for it stood in immediate contradiction to his necessary conviction in daily life, whereby he was bound to regard himself as free and independent.
Gist of Idea
Spinoza could not actually believe his determinism, because living requires free will
Source
Johann Fichte (works [1798], I:513), quoted by A.W. Moore - The Evolution of Modern Metaphysics 06.2
Book Reference
Moore,A.W.: 'The Evolution of Modern Metaphysics' [CUP 2013], p.149
A Reaction
This seems to be invoking Kant's idea that we must presuppose free will, rather than an assertion that we actually have it.