I hope you remember the spoon scene in The Matrix. — Copernicus

A crap film.

Exactly. — Copernicus

It's a performative contradiction.

How am I related to the chicken? — Copernicus

Ah! There's the proof! He denies it again!

Even if - and I want to make this perfectly clear - even if there is no obvious relation between you and the chook, that does not say that there is no relation.

Just as you say all our deeds are selfish.

there is no relation. — Banno

If you meant from the aspect of causality (butterfly effect), then sure, we're related. But if you meant uniformity like electrons, then you're missing the point.

performative contradiction. — Banno

It's not.

"I'm sure everything is unsure" = Everything is unsure.
"I'm unsure if everything is unsure" = Everything is unsure.

@Copernicus

Does the relationship between whatever a "self" ought to reference and the "body" (of that self) have any relevance to the opinion that "all acts are selfish acts"?

There is sometimes the anomaly of conjoined twins who share a composite body, like Abby and Brittany Hensel. Maybe we would say two selves (individuals with distinct personalities, minds) share one body, while one controls left side and the right side.

In accordance with your OP, they both are always acting in their individual self interest all the time. Mutual coordination/agreements, trade-offs for the shared self (as shared body), is just a "refinement of selfishness". In one sense they share a self and another they don't.

This same kind of relationship extends to people who do not share a body in the conventional sense, but are embedded in ecological environs/processes (dependent functional relationships between each other that comprise "organizations").

How do we assign what belongs to self (as body, or otherwise) and what doesn't in terms what goes into instantiating whatever the "self" is?

Someone could do an act and not really know what caused them to act. They may form a rationalization/narrative that explains their action, but the motivation could actually be caused by something completely unknown to them. Suppose a benign brain tumor enhances the reward for gambling in a person towards self-destruction. For the person the desire/reward of gambling might align with their desire/will at one point in time and be against their desire/will at another. They know with respect to a future condition they'd be better off if they could inhibit their impulse yet they always succumb to the act.

What goes into instantiating the self goes beyond the limited awareness of any self.

Well-structured post. Still, I agree with several critiques made by @Count Timothy von Icarus, @Mijin and @Banno. The argument equivocates between different senses of "selfish" for its plausibility, and it does not take into account the intent or aboutness that I believe is highly relevant for determining if an act is selfish or not.

The mind is inherently solipsistic — it perceives the world only through itself. Every thought, feeling, or impulse is filtered through the self before it can be acted upon.

Thus, when a person helps another, the cause is not the suffering of the other itself, but the internal feeling of empathy, duty, or moral satisfaction that drives them to act. The ultimate motivation, therefore, always resides within. — Copernicus

This is questionable. It seems trivially true that our thoughts, feelings, perceptions, and impulses are dependent on our minds, but that does not mean that our thoughts, feelings, perceptions and impulses are about our minds. One could argue that a person can be motivated to relieve another person's pain by perceiving a person in pain and then having certain emotions and beliefs in response that makes them act without this making their act selfish. Those defending altruism typically don't reject that a mental cause plays a role in a person making an intentional action. So I don't see how our ultimate motive being selfish follows.

Something can be caused without that cause being what that something is about. To take a photo, light that bounces off objects must be filtered by the internal mechanism of the camera, but that does not mean that the photo is about the camera's internal mechanism.

The "causal view" of aims has strange implications about ultimate motivation. Why stop at internal states if motivation is merely based on causal connection? My parents having sex plays a causal role in my current act of writing these sentences, but I don't think that means that my ultimate motivation in writing these sentences is about my parents having sex.

Psychological studies confirm this. Acts of charity, generosity, and volunteerism are correlated with activation in the brain’s reward centers (ventral striatum, medial prefrontal cortex). Helping others feels good, biologically. The altruist experiences hormonal reinforcement through dopamine and oxytocin — demonstrating that “good deeds” literally reward the doer.

This blurs the line between altruism and pleasure: the altruist helps others because it pleases him to do so. — Copernicus

Such acts may be correlated with activation in the brain's reward centers, but that does not show that having a pleasurable experience is the agent's aim. Sweating is correlated with me jogging, but that does not mean that my aim with jogging is to sweat. "Pleasure" may be a side-effect (even a necessary one) of other-regarding actions without collapsing into being self-regarding.

whatever the "self" is — Nils Loc

Yes, we'd need a standard definition for "self".

Two straight lines can't intersect at more than one point. No act can be selfless.

There's no way around them. Some things are just like that (at least to our comprehension), like causality.

I see. Are you saying that your claim is true by the definition of the concept you're using? And you call this concept "selfishness" or do I misunderstand you?

You can use any symbols and sounds you want for the concept you're talking about, of course. However, why should others use "selfishness" for that concept? The term typically carries a cynical meaning. It is often used to critique individuals for failing to properly account for the interests of others because they focus on their own interests too much. For instance, when a person is told "you're being selfish!" it usually implies that the person is acting with insufficient regard for others. So the term your concept uses is normatively loaded due to this association.

When you write that selfishness is the foundation of all motivation, it sounds like you're making the cynical and controversial claim that human actions are always self-regarding, often at the expense of the interests of others. If you're merely stipulating the term "selfish" to cover all intentional action, you are free to do so, but I'm not sure what the utility of it is.