Scientists and other people who believe this are wrong. For a detailed validation of free will, see my book The Illusion of Determinism: Why Free Will Is Real and Causal.
Determinists claim that everything you believe, want, do, and feel is determined by factors outside your control. Radical determinists say that everything you did today was pre-determined by a chain of causal laws starting at the time of the Big Bang, thirteen billion years ago. Talk of being a meaningless speck in the universe!
What is the error? Determinism entails a form of self-contradiction. Let me start with a 1986 quote from a world-famous physicist, the late Stephen Hawking:
Yet if there really is a complete unified theory [of physics], it would also presumably determine our actions. And so the theory itself would determine the outcome of our search! And why should it determine that we come to the right conclusions from the evidence? Might it not equally well determine that we are drawing the wrong conclusion? Or no conclusion at all? (pp. 20-21)
(It bears nothing that Hawking did not have a viable solution to the dilemma, as I point out in my book, pp. 81-81).
The determinist believes, as Hawking says, that humans are powerless to make choices. This means that their belief in determinism was forced on them by causes beyond their control. They could not help believing it. This also means that forces beyond their control also determine the beliefs of anyone who disagrees with them. The debate becomes a contest between the equivalent of robots. This is a dead end. If determinism is true, we could never validate anything. The belief in determinism could never be validated because nothing could be validated.
Can we identify and validate what free will consists of? Freedom of action is not too far from the definition, but it is not the most fundamental choice. Let’s look beyond freedom of action to determine what causes action itself, above the level of one’s automatic bodily processes and automatic sense perception.
The answer is the rational faculty—that is, the choice to engage in conceptual thought. Thinking requires hardware and a brain. It must be stressed that thoughts and neurons are not identical. Physical entities can be colored and have mass, shape, and extension. Thoughts have none of these attributes. Ideas or thoughts can be unclear, confused, scary, or logical. Neurons have none of these attributes. The two cannot be identical even though consciousness depends on, that is could not exist, without the brain.
Consciousness as such is best viewed as an emergent property of the brain and nervous system. It exists on two levels: sense perception and conceptualization. Sense perception is not volitional; it is the automatic result of sensory material being processed by the brain. The process is subconscious, but the result is awareness of some aspect of reality.
Volition emerges at the conceptual level. Concepts are formed by integrating the material of the senses. For example, one can observe that certain structures have walls, a roof, doors, and often windows, and that people live in them. To form the concept of house, one ignores (abstracts out) the fact that the various attributes exist but differ in amount (in size and shape), and then one gives the concept a name and a definition (a structure serving as a dwelling). This is the basis of language. More complex concepts integrate lower-level concepts. Concepts can be integrated into thoughts and reasoning. What is unique to the conceptual or rational level of consciousness is that it is turned on or off by choice. Free will is the choice to think or not to think. It is not automatic; it requires self-generated effort (Objectivism: The Philosophy of Ayn Rand).
You can validate free will easily. Pick up a book and choose to start reading. At some point you may find that you drift off; you lose focus, zone out. But you have the power to notice this, to decide whether you really want to understand what you are reading, and if so, to re-focus. Or you may decide what you are doing is not worth the trouble and turn to something else. You can also choose not to think about what you are reading because, even though you know it is important, it makes you feel anxious. This is evasion. Evasion, too, is a choice.
It must be noted that free will as such cannot be proved. Why? Because free will is a precondition for proving anything. Robot regurgitations are not knowledge. Free will is, thus, an axiom of epistemology. It is self-evident to introspection and cannot be refuted without contradiction.