Full Idea
Perhaps the 'I' in 'I think' no more serves to pick out a certain object than does the 'it' in 'it is raining'.
Gist of Idea
Perhaps 'I' no more refers than the 'it' in 'it is raining'
Source
E.J. Lowe (Introduction to the Philosophy of Mind [2000], Ch. 2)
Book Reference
Lowe,E.J.: 'Introduction to the Philosophy of Mind' [CUP 2000], p.13
A Reaction
A nice example to remind us that not all English pronouns have genuine reference. You could reply that 'it' does refer, to the weather; or that you can switch to 'you think', but not to 'they/we are raining'.