Free will is usually defined by three conditions: the ability to do otherwise; control of one’s own choices; responsiveness to reasons. The compatibility of free will with determinism lies at the heart of the philosophical debate at the metaphysical level. This debate, while being increasingly refined, has not yet reached a conclusion. Recently, neuroscience and empirical psychology have tried to settle the problem of free will with a series of experiments that go in the direction of so-called illusionism: free will (...) as the conscious control of our behavior cannot exist, being a mere illusion. But even in this case, the experimental results are challenged at various levels. Considering that in most moral and legal systems, the subject’s liability derives from their freedom, the usefulness of preserving the concept of freedom – which incidentally responds to a very strong commonsensical intuition – suggests the need for an operational solution. This could be done by resorting to the concepts of capacity and cognitive control, which are measured by a set of well-established neuropsychological tests. Our preliminary proposal is to create an index, the first step towards a specific quantification and measurement of free will, to be used especially in ethical and legal contexts. Theoretical premises, practical difficulties and objections to this approach are also discussed and addressed. (shrink)