It is undoubtedly true that a color appears to us only when we have a presentation of it. We cannot conclude from this, however, that a color cannot exist without being presented. Only if the state of being presented were contained in the color as one of its elements, as a certain quality and intensity is contained in it, would a color which is not presented imply a contradiction, since a whole without one of its parts is indeed a contradiction. But this is obviously not the case. Otherwise, it would also be absolutely inconceivable how the belief in the real existence of physical phenomena outside our presentation could have, not to say originated, but achieved the most general dissemination, been maintained with the utmost tenacity, and, indeed, even been shared for a long time by the most outstanding thinkers. Bain said: "We can think of a tree as perceived, but not as unperceived. There is a manifest contradiction in the supposition". If what he said were correct, his further conclusions could not be objected to. But it is precisely this which cannot be granted. Bain explains this statement by remarking, "We are required at the same moment to perceive the thing and not to perceive it". It is not correct, however, to say that such a demand is placed upon us, for, in the first place, not every act of thinking is a perception. Secondly, even if this were the case, it would only follow that we can think only of trees that have been perceived by us, but not that we can think only of trees as perceived by us. To taste a piece of white sugar does not mean to taste a piece of sugar as white. The fallacy reveals itself quite clearly in the case of mental phenomena.
(...) It is not correct, therefore, to say that the assumption that there exists a physical phenomenon outside the mind which is just as real as those which we find intentionally in us, implies a contradiction.
Descubro para mi sorpresa que Brentano ha dado exactamente la misma respuesta al problema que la que ofrecí yo hace dos posts. Creo que voy por el buen camino.