The recent publication of Freud’s correspondence to his school friend, Edward Silberstein, has provided new impetus for research into Freud’s relationship with the philosopher Franz Brentano. In this paper I will address one possible objection to any claim that the philosopher could have influenced Freud on a theoretical level. It may be argued that there could be no significant theoretical influence because the psychoanalyst constructed a model of mental functioning which presupposes an unconscious, while Brentano was a philosopher of consciousness, who denied the very existence of unconscious ideas. I will demonstrate that, despite his rejection of unconscious mental functioning, Brentano presents a systematic investigation into what he perceives to be the strongest arguments in favour of the existence of unconscious ideas. Although he finds each account to be flawed, Brentano frequently offers a possible corrective, suggesting certain conditions as principles which must be observed by anyone hoping to formulate a reasonable thesis to support the existence of unconscious ideas. I argue that it is this analysis which helped Freud to formulate a coherent account of the unconscious which does not fall prey to the objections Brentano levelled against preceding conceptions of unconscious mental processes.