Hardcore determinism (which is to be distinguished in degree from "compatabalism", since compatablism holds that free will is compatible with determinism) denies that there is such thing as a genuine free choice. It proposes that the choices of human beings are pre-determined; that they are entirely causal. Determinists believe that biology and physics is the sole determiner of all human actions. They take the narrow view that, to use an example, the action of someone typing on a computer is pre-determined because it's governed by biological chemicals and signals within one's body, from one's brain and through one's nerves. To the determinist, there is only one possible future at all times, rather then multiple probabilities that can be effected by people's deliberate choices. In effect, they believe that we are all the sock puppets of biological forces.
Determinism simply turns biology into a god, replacing a religious deity as the determiner with "gaea" itself. Instead of the theory that "god predtermines all of my actions", it's the theory that "biology determines all of my actions". But to me, this is silly; it fails to distinguish between actions that are purely biological (like taking a piss, breathing, heart beating; things that one cannot avoid doing) and ones that are not (like what kind of music to like, what brand of shoes to buy, what job one takes up). People's preferances cannot be adequately explained by biology or psychology for that matter. Indeed, contemporary psychology has done little to nothing to advance understanding of what determines people's preferances. It is often marxist in that it is "culturally determinist", holding that people's preferances and ideologies are entirely pre-determined by whatever culture, economic status or nation that they just so happen to have been born into.
Determinists fail to make a proper distinction between the faculties and the application of the faculties. It presumes that the faculties predetermines one's choices. But within the context of any given faculty, there is variance as to how the individual applies their faculties. Yes, one's faculties are pre-determined biologically, but how one exercises them is not. A genuis can choose to not exercise their intelligence, and a dope can try to push the limits of their abilities. A strong man can choose not to exercise their strength, and a weak man can choose to push their physical abilties to their limits. Faculty does not automatically translate into action or use. People can choose or not choose to use their faculties, and that has a profoud effect on both biological and social evolution.
The constraints of the physical world does not negate the existance of choice. There is "headroom" within the natural forces. Yes, obviously someone can't just "wish" their own reality into existance, they can't abolish the laws of nature. The point is that, within the confines of reality, they actually have choices. Determinism makes the error of assuming that constraints of natural forces makes choice not exist within that framework. The laws of nature in themselves allow for me to have choice. They give me a degree of physical control over my environment. The individual can use nature to their advantage (simple example: by cutting down some trees and building a home). Free will is a consequence of those very laws that determinists think negates it.
Gravity is a natural law, as is inertia. Obviously, I can't "will" gravity away. Where choice comes in is how I use gravity. From experience, I know that if I jump, I will fall back to the earth. That doesn't mean that I have no choice. It means that my choices must be confined to physical laws. I can use those physical laws in ways that I desire, without breaking them. I can use gravity and inertia to achieve ends that I desire, including the design of technology. I can learn to control gravity and inertia as to, say, dribble a basketball. That wouldn't negate gravity, but I'd still be exercising purposeful action. Simply, the existance of physical laws does not negate choice. When people learn the consequences caused by various courses of action determined by physical laws, they purposefully change their behavior to adapt to it, and they come up with ways to use those physical laws to achieve consequences that they desire.
The simple observation of the many instances where people change their course of actions in the face of negative consequences indicates a capacity to choose among different courses of action; and it indicates, as praxeology does, that individuals purposefully seek to attain ends in order to remove a source of disatisfaction. To use an example, suppose someone pricks their finger on a sharp object. They gain the experience of pain from this. Over time, with this new experience of pain having been felt, they will change their behavior as to avoid the pain, and therefore either not mess around with sharp objects anymore, or find ways to minimize or avoid the pain involved with them. To say that such a change in behavior is pre-determined is to ignore the consequences of experience.
The existance of stupidity does not negate choice. Someone who is stupid lacks experience, the very factor that determinists gloss over. The person who pricks their finger on the sharp object for the first time, who has never ever seen a sharp object before then, lacks the experience. On the other hand, someone who does have such experience, but chooses to harm themselves anyways, is ignorant, not stupid. The ignorant person does possess the capacity to change their behavior, but chooses not to use it. As the saying goes, to choose to do nothing is still to make a choice. Ignorance is a choice, not something that is unavoidable. Ignorance is a lack of adequate application of one's faculties.
Humans are capable of supressing their own instincts; this is precisely what free will involves. They can commit suicide and thus resist the instinct towards staying alive, they can become celibate and thus resist the instinct towards procreation, and so on. People can develope tolerance thresholds for pain, as to overcome their instincts to coil from it; some people even find pleasure from pain. To suppose that humans are completely bound by instinct is to ignore precisely what distinguishes human beings from other species. Furthermore, it inevitably must deny that there is such thing as personal responsibility, for every act a person made must be considered inevitable and instinctual. Determinism, in effect, denies human reason, while similtaneously contradicting itself in using human reason to try to explain things in deterministic psysiological terms.
Viewed from a historical perspective, determinism implies that whatever the current situation is in the world we must accept as inevitable, as if nothing could have been done in the past to change it. This ignores any possibility for alternative courses of action. Furthermore, this inevitably also implies that there is nothing we can deliberately do with respect to the future; that the future cannot be effected by our choices in the present, because our choice in the present are considered to be only one-way, without any variance in possibilities. A deterministic view of history can be neither progressive (future-oriented outlook towards change) or romantic (past-oriented outlook toward moving things back to the way they were before). It must remain present-oriented, and consider the status quo as inevitable. It therefore can have no spirit towards any kind of change at all if taken entirely seriously. It aknowledges that changes occur, but refuses to aknowledge that we have any real control over them at all.
A determinist engaging in arguementation has refuted themselves already by engaging in a performative contradiction. The determinist cannot claim to have chosen to believe in determinism in the first place, and in trying to convince others of determinism they are conceding that people can freely choose, and they are themselves engaging in a free choice. Argueing against free will is like argueing against self-ownership by using your self-ownership; it's self-contradictary on impact. Argueing with a determinist is like argueing with a brick wall. Arguementation between two people, in pure determinism, would be no different than expecting two rocks to agree with eachother. An arguement between person A and person B would be no different than two answering machine recordings being played to eachother. It's an absurdity.
Hardcore determinism is mistaken in its understanding of human nature. While it is true that humans cannot choose to do things that are physically impossible, that some options are simply impossible within the context of human capability, that there are definite limits on human capability and that everyone's perception is fallable, it does not follow that biology, physics and geology dictates everyone's choices in the absolute. They are merely factors in the overall equation, not to be ruled out entirely, but neither to be considered the be all and end all. While someone cannot choose where they are born, they are able to make choices among various ends within whatever environment they are born into. There is a diversity of possibilities within any given environment. Environments do not uniformly dictate everything to the individual. The individual can take advantage of their environment in a variety of ways.
Someone's economic status does not uniformly determine what their opinions and behavior will be. Someone's ethnic origin does not uniformly determine what their opinions and behavior will be. Someone's national origin or citezenship status does not uniformly determine what their opinions and behavior will be. These environmental conditions not only are dynamic in themselves, and therefore change over time, but the individual within such environments can be responsible for creating and changing them, or even leaving them entirely. Cultural and economic determinism is nonsensical in that it supposes that people's behavior can be entirely pre-determined by something that is in reality determined by those people's behavior. It has it all reversed. Furthermore, it relies on something that is unstable in itself (national boundaries and economic statuses are dynamic things) as the basis for what everyone is supposed to be pre-determined by. How can people be determined by something that is in itself never the same at any given moment, I.E. cannot be determined in any static sense?
People have argued against free will for ages. Their arguements have failed time and time again, because they implicitly accept it and experience it all the time. It is often a cheap way for someone to avoid assigning responsibility either to themselves or to others. Determinism is ultimately incompatible with freedom. An action cannot be both voluntary and pre-determined at the same time. If it is pre-determined, then it is not voluntary, it is unavoidable, like death. If it is voluntary, then it is not pre-determined.