Concluding on William James's Psychology, the Briefer Course (1892). We briefly cover emotions and spend the bulk of our time on will.
Talking about emotions allows us to refresh on James's overall theory of mind: An organism's activity basically involves various signals coming in through the senses and going out to produce reaction. This chain can reach to a greater or lesser extent into consciousness: from a reflex (mindless) action at one extreme, which might be or simply start as an instinct, or might have once been a choice but then became a habit. An emotion is a physiological reaction that we then notice; this feeling of the physical change is the emotion. Attention (accompanied by memory) is the faculty that filters the confusion of the stream of experience into identifiable things. A voluntary action is one taken with a memory that such an action can be done (so it must have been done the first time by instinct, reflex, or chance) with a plan to produce the same effect.
This gives a partial picture of what "will" amounts to. It's not a single faculty, but a combination of different phenomena, mostly related to attention. You could consider every voluntary action to be "willed," but many such actions don't hold our attention more than a moment or are part of a habitual chain, such that they don't actually attract our attention unless something goes wrong. I might be thirsty and reach for the water in front of me while concentrating on something else. Did I "will" the action? Well, the only times we generally use that term is when there's some difficulty in the decision or the action: I reach for the water, but I'm very tired and have to overcome my tiredness to reach for it, as if the obstacle is saying "are you sure?" Or maybe the drink is whisky and I stop to think whether I’ve already had too much.
In short, these input-output chains can conflict, which then brings attention to bear, and when we do make a decision, that’s where the explicit “fiat” that we typically call “will” comes in (even though many more actions are voluntary and hence willed as opposed to against-your-will). James thinks that whichever idea is held in attention the longest tends to have causal efficacy. If my desire for the alcohol is great, then repeated acts of attention would be required to pull myself back from that desire and consider the idea that enough is enough. Insofar as we have to exert this kind of effort, that really feels like will.
With this picture established, we spend some time bickering about James’s stance regarding free will. He explicitly says that he’s not going to deal with that in this essay, that we can still talk about moral psychology with just the facts of temptation and effort as described above, and we can judge someone’s virtue from their apparent capacity to summon up such effort. As a scientist, James says that he’s committed to the idea that all actions have a physiological cause, yet he can distinguish phenomenologically between actions that seem like involuntary, reflex actions from ones that seem deliberative and voluntary. But are we really, metaphysically, free? The scientist must remain silent on this point, as metaphysics is outside the scope of science.