Like Fichte, Brentano had one simple and powerful insight. He declared: there a basic difference between a mental and physical act. if I slip on the snow and fall flat on my back, that is an physical act. When I think, I have to think something; I have to focus my mind on it. You could compare all mental acts (thinking, willing, loving, trying to remember something) to a searchlight beam stabbing into the darkness. There is an element of will, of 'intentionality,' in all mental activity. So it is quite inaccurate to compare mental activity to chemistry, or to a kind of drifting, like leaves on a stream. It flows purposefully or not at all.
Colin Wilson