Is it really possible for someone to engage in a completely selfless act? A rational exploration of the question must lead to a negative rather then affirmative answer. For isn't it the case that no matter what action one engages in, it involves their selves and some kind of motivation on their part? So long as the individual in question could be said to have a motivation for acting, even if this motivation is a benevolent one, it cannot be said that they are acting in a truly selfless manner. So long as one has consciousness, so long as one is self-aware, one cannot truly be selfless. In order to be selfless one would have to cease to exist altogether, or by the very least enter an inhuman state in which one has no volition. But such a state of being is not how human beings work. It could only be used to describe non-conscious forms of life such as a plant.
Altruism is not an objective description of human actions so much as a rationalisation used to compel people into engaging in certain actions. It is certainly true that one could engage in an action that benefits another rather then oneself, but such an action could not take place without the deliberate conscious effort and motivation of the individual in question. If a benevolent act towards another is truly voluntary, then it cannot be said to be genuinely altruistic because in such a scenario the individual actually perceives the act to be in their self-interest at least on a psychological level. That is, they desire to give to others. A genuine desire originates within the individual themselves. Satisfaction is obtained upon the fulfilment of the desire, even if the desire is to fulfil a perceived positive obligation towards another.
So long as human beings act to remove a source of dissatisfaction, it could not be said that they act in a void of self-interest. They act in the pursuit of satisfaction, which is their perceived self-interest. They employ means for the purpose of obtaining desired ends. The statement that humans act in self-interest on a fundamental level and that human beings are rational animals does not mean that humans will always make correct choices, that the ends they desire are necessarily logical and ethical, or that the means they employ in the pursuit of such ends are the proper or most efficient ones toward obtaining their goals. It is merely a description of how human action works, that human beings are volitional creatures with goals and the capacity to choose among means for the purpose of obtaining their goals.
On one hand, every single person is rational in the sense that they possess the faculty of reason and are self-aware. They have the ability to freely make choices. In this sense of the word, no one can be more rational then anyone else because this is merely a description of our fundamental natures. On the other hand, in terms of their actual beliefs and choices, no one is consistently rational if we are using rational to mean in accordance with objective reality and their actual best interest. People make all sorts of choices that can easily be demonstrated to be harmful to them, and people believe plenty of things that are not in accordance with objective reality. In this sense of the word, some people are simply more rational then others, make more coherent arguments and better choices. But when libertarians describe human beings as inherently being rational, we are using the first sense of the word, not the second. It would be disingenuous to act as if we are arguing that everyone is consistent in their beliefs, sharp as a bell and makes wonderful lifestyle choices. Altruism cannot be a logical description of human action because it contradicts the fundamental nature of how humans act. That is, no rational agent, in the general way in which rationality is defined, can possibly act in a manner that is entirely detached from motivation or desire. No human being is actually an altruist precisely because they are human beings. It would seem to be the case that the insights of praxeology and psychological egoism demonstrate this beyond the shadow of a doubt.