Full Idea
The proposition 'I think' was useless to Spinoza, because it expresses a merely contingent proposition, where certainty must be founded in necessity, and because it refers to the first person, when truth comes from rising above our own mentality.
Gist of Idea
'I think' is useless, because it is contingent, and limited to the first person
Source
report of Baruch de Spinoza (The Ethics [1675]) by Roger Scruton - Short History of Modern Philosophy Ch.5
Book Reference
Scruton,Roger: 'A Short History of Modern Philosophy' [ARK 1985], p.51
A Reaction
I find both of these criticisms very appealing. One might simply say that the starting point of philosophy is not the process of thinking, but the contents of thinking. Descartes' move is like astronomers becoming obsessed with telescopes.