"Imagine someone has seen a flower and seeing this flower has left the person with a passive affect - joy in the flower's beauty. This affect might be so positioned in the mental mechanism as to move the person toward the flower to pick it. The person is conscious of the flower's beauty and conscious of the desire to move to pick it. Indeed, he will likely also be conscious of his picking it, and so on.
"Now let us ask this person why he picked the flower. He may be unconscious of the fact that his joy in the flower's beauty is what moved him. He might think of his idea of the flower's beauty as merely a passive representation of beauty and not an affect capable of causing him to move. In reflecting on his mind, he sees the representation of the flower's beauty, attended by a desire to pick it, followed by the motion to pick the flower. He does not see that the representation, being an affect, caused the subsequent motion.
"The affectivity account [of consciousness] can explain this phenomenon perfectly. The idea of the flower, by itself, does not cause the motion - that is, the fact that this idea moves him is not merely a feature of the essence of this idea. Instead, the idea moves him to act - that is, it becomes an affect of desire - in virtue of the causal relations it bears to the other constituents of the mental mechanism. As the idea exercises its power, it has a certain effect on the conatus that has a parallel effect in the body that results in a motion toward the flower. But these causal relations between the idea and the affect of desire are not themselves affects and thus not the objects of conscious awareness. That is, we might be aware of our judgment of beauty and we might also be aware of our desire to pick the flower, followed by our motion toward the flower to pick it. But we might not be aware that the idea is an affect, and that this affect causes the motion. In other words, we might conclude that nothing caused the desire apart from us; we might therefore conclude ourselves to have initiated the motion toward the flower. In short, Spinoza takes ideas and affects to interact in the mental mechanism in the mind in such a way as to cause human behavior, though he also asserts that our conscious awareness of the workings of this mechanism is often quite limited.
"Another way to conceive of this illusion of free will is in terms of explanation. Consider a passive desire I might have, say, to have a slice of pie. My desire for the pie is explained by its adequate or total cause. Let's say that the adequate cause of my desire for pie includes my hunger, the passive passion of love that arises in me when I view the pie, and the constitution of the pie in virtue of which it can cause this desirous love in me. The cause of this inadequate idea/passive affect thus includes modes outside of my mind and body; I am only a partial cause of this desire. If an idea in my mind is partially explained by ideas not present in my mind, then the idea in my mind is inadequate - I lack knowledge of some part of its cause. That is, were I to reflect on my desire for the pie, I could not offer a complete explanation for my desire; the ideas available to me could not adequately explain my movement toward it. Given the partial explanation available to me, I might not feel that the resultant desire has been necessitated; as far as I know, the cause does not require it.
"This is just how Spinoza distinguishes between our ideas of necessity and possibility. He says, 'I call the same singular things possible, insofar as, while we attend to the causes from which they must be produced, we do not know whether those causes are determined to produce them' (4def4). In the case of an inadequate idea or passive desire, we do not know the total cause of the mode of thought in question. Thus, from what we know of the cause, we do not know whether the effect is necessary, so we take it to be merely possible. Because we cannot know the total cause of a passive desire; we take it and the resulting action to be merely possible, which might lead us to conclude, falsely, that we have free will. This error is only possible when we do not have a conscious awareness of the adequate cause of our desire, i.e., the error is only possible in inadequacy and not when we act from adequate knowledge." - Eugene Marshall, The Spiritual Automaton: Spinoza's Science of the Mind