There was once a time when philosophers marvelled that man could move his limbs even though he was not familiar with the nerve and muscle processes on which such movements depended. They even went so far as to conclude that man was quite incapable of moving his body by himself. Whenever he wished to perform some movement, they believed, a higher power had to come to his aid and do it for him. (Schlick, General Theory of Knowledge, Blumberg, tr., Open Court [La Salle, IL: 1974] p. 1)
One does not go to logical positivists for competent history of philosophy, but this idea with which Schlick starts is not made-up, although the vagueness of it makes it at least sound exaggerated, since it's only one lineage of philosophers who reasoned in this way. The most influential proponent of the idea described here is Nicolas Malebranche, since this is one of Malebranche's arguments against second causes, and his major argument against the idea that the mind causes the body to move. We find an example of this kind of argument in the Fifteenth Elucidation to The Search after Truth:
But I deny that my will is the true cause of my arm's movement, of my mind's ideas, and of other things accompanying my volitions, for I see no relation whatever between such different things. I even see clearly that there can be no relation between the volition I have to move my arm and the agitation of the animal spirits, i.e., of certain tiny bodies whose motion and figure I do not know and which choose certain nerve canals from a million others I do not know in order to cause in me the motion I desire through an infinity of movements I do not desire. (LO 669)
I deny that there is a relation between our thoughts and the motion of matter. I deny that the soul has the least knowledge of the animal spirits of which it makes use to move the body it animates. Finally, even if the soul had an exact knowledge of the animal spirits, and even if it were capable of moving them, or of determining their motion, I deny that it could thereby select the nerve ducts, of which it has no knowledge, in order to impel the spirits into them and thus move the body with the promptness, exactness, ad force observed even in those who least know the structure of their body. (LO 671)
For, in short, since the soul is a particular cause and cannot know exactly the size and agitation of an infinite number of particles that collide with each other when the spirits are in the muscles, it could neither establish a general law of the communication of motion, nor follow it exactly had it established it. Thus, it is evident that the soul could not move its arm, even if it had the power of determining the motion of the animal spirits in the brain. (LO 671)
When you will to move your arm, you do not will the micromotions that are required for your arm to move, because you have no knowledge of them. All you do know is that you will, and the motion follows, unless something interferes. The motion operates entirely by 'a general law of communication of motion'; and laws of nature, in a Cartesian system, are acts of God.
Ironically, part of Malebranche's overall argument is that descriptions like 'the union of the mind and body' and 'the will's power to move the body' are associated with no clear idea and therefore are merely "vague and indeterminate words" that are used by prejudice so that those who use them do not know what they are saying, which is very much a forerunner of the logical positivist attack on metaphysics as meaningless.
Hume takes up Malebranche's argument in ECHU, Section VII (which repeats a significant number of Malebranchean arguments for occasionalism):
Here the mind wills a certain event: Immediately another event, unknown to ourselves, and totally different from the one intended, is produced: This event produces another, equally unknown: Till at last, through a long succession, the desired event is produced. But if the original power were felt, it must be known: Were it known, its effect also must be known; since all power is relative to its effect. And vice versa, if the effect be not known, the power cannot be known nor felt. How indeed can we be conscious of a power to move our limbs, when we have no such power; but only that to move certain animal spirits, which, though they produce at last the motion of our limbs, yet operate in such a manner as is wholly beyond our comprehension?
He later notes the positive Malebranchean proposal (in order to reject it):
In like manner, it is not any energy in the will that produces local motion in our members: It is God himself, who is pleased to second our will, in itself impotent, and to command that motion which we erroneously attribute to our own power and efficacy.
Hume thus accepts Malebranche's negative argument but not his positive solution, a recurring pattern in Section VII, which is heavily influenced by Malebranche's arguments for occasionalism. It's notable that in describing the Malebranchean position, Hume also uses the generic term 'philosophers' (that it is Malebranche's is mentioned only briefly, in passing, in a footnote at the end), as he has occasionally does throughout the Enquiry; it's a tempting hypothesis that Schlick's generic 'philosophers' is an imitation of this habit, particularly as Schlick is raising the point in order to argue that there is a flaw in skepticism.
Since it's unlikely that Schlick ever read much Malebranche himself, the only source Schlick seems to have other than Hume is Arnold Geulincx. Geulincx may actually have the priority over Malebranche as the originator of the argument, but given the late posthumous publication of most of Geulincx's works and Malebranche's general contempt for non-Catholic philosophers, it's difficult to imagine any serious influence. In any case, Geulincx's notion is summed up in his famous phrase, Quod nescis quomodo fiat, id non facis, which might be colloquially translated as, "If you don't know how you're doing it, you're not doing it." Given that this idea is more or less what Schlick will diagnose as the false assumption of skepticism about knowledge, and that Schlick will mention Geulincx twice by name later in the work, it makes sense that Geulincx would be specifically in mind here. Geulincx had been revived by authors of works about the history of philosophy and for a while was much better known than he is today.
