The persistence of the free will problem in philosophy seems to me something of a scandal. After all these centuries of writing about free will, it does not seem to me that we have made very much progress.
(Freedom and Neurobiology, p.37)

Two centuries ago, Immanuel Kant called it a scandal that academic philosophers were so out of touch with the common sense of the masses when they doubted the existence of the external world. David Hume had criticized the Theory of Ideas of his fellow British empiricists John Locke and George Berkeley. If they are right that knowledge is limited to perceptions of sense data, we cannot "know" anything about external objects, even our own bodies. Kant's main change in the second edition of the Critique of Pure Reason was an attempted refutation of this idealism (B 273). He thought he had a proof of the existence of the external world. Kant thought it a scandal in philosophy that we must accept the existence of things outside ourselves merely as a belief, with no proof. He said that "speculative metaphysics must be investigated

to prevent the scandal which metaphysical controversies are sure, sooner or later, to cause even to the masses.

However innocent idealism may be considered with respect to the essential purposes of metaphysics (without being so in reality), it remains a scandal to philosophy, and to human reason in general, that we should have to accept the existence of things outside us (from which after all we derive the whole material for our knowledge, even for that of our inner sense) merely on trust, and have no satisfactory proof with which to counter any opponent who chooses to doubt it. (Preface to Second Edition, Critique of Pure Reason)

Martin Heidegger commented on Kant's scandal:

The "scandal of philosophy" is not that this proof has yet to be given, but that such proofs are expected and attempted again and again.
(Being and Time, Macquarrie and Robinson, translators, pp. 249)

The free will scandal is related to the knowledge scandal because so many philosophers and scientists have thought that they could prove that free will does not exist, that it is an illusion.

Ths most common proof is based on the two-part standard argument against free will.

If our actions are determined, they are not free.

If our actions are directly caused by chance, they are simply random and we cannot be responsible for them.

Despite more than twenty-three centuries of philosophizing, most modern thinkers have not moved significantly beyond this core problem of randomness and free will - the mistaken idea that free actions are caused directly by a random event.