From Will Durant's The Story of Philosophy:
"Almost without exception, philosophers have placed the essense of mind in thought and consciousness; man was the knowing animal, the rational animal. 'This ancient and universal radical error [says Schopenhauer] must before everything be set aside.' 'Consciousness [Schopenhauer continues] is the mere surface of our minds, of which, as of the earth, we do not know the inside but only the crust'. Under the conscious intellect is the conscious or unconscious will, a striving and persistent vital force, a spontaneous activity, a will of imperious desire. The intellect may seem at times to lead the will, but only as a guide leads his master; the will 'is the strong blind man who carries on his shoulders the lame man who can see.' We do not want a thing because we have found reasons for it, we find reasons for it because we want it; we even elaborate philosophies and theologies to cloak our desires. Hence Schopenhauer calls man the 'metaphysical animal': other animals desire without metaphysics. 'Nothing is more provoking, when we are arguing against a man with reasons and explanations, and taking all pains to convince him, than to discover at last that he will not understand...' Hence the uselessness of logic; and even logicians use logic only as a source of income."
"Consider the agitated strife of men for food, mates, or children; can this be the work of reflection? Certainly not; the cause is the half-conscious will to live, and to live fully. 'Men are only apparently drawn from the front; in reality they are pushed from behind"; they think they are led by what they see, when in truth they are driven by what they feel---by instincts of whose operation they are half the time unconscious. Intellect is merely the minister of foreign affairs; 'nature has produced it for the service of the individual will. Therefore it is only designed to know things so far as they afford motives for the will, but not to fathom them or to comprehend their true being.'"
The will to be rational?
The will, however, like human consciousness itself, is not something the surgeon can poke around inside the brain and actually locate. She can't point to a spot and say, "this particular mass of cells here is the human will." Or, further: "you will notice how its shape, size and color is clearly distinguishable from the shape, size and color of the consciousness in which it is embedded; which is clearly distinguishable, in turn, from the rest of the brain".
Thus there is no way, of course, that Schopenhauer's argument can be confirmed empirically or "scientifically". We can't know for certain if what he suggested is, in fact, true. We can only extrapolate from our sensations, perceptions and what our thinking minds collate into conceptions and then offer what some will or will not construe to be a reasonable argument.
Does it seem reasonable to you? It certainly seems reasonable to me. Consider, for example, that life on earth has been evolving for literally millions and millions of years. And for the overwhelming preponderance of life-forms, metaphysics hardly ever came up at all. The human species, on the other hand, has evolved a brain that allows it ponder the world "philosophically". It is a brain that allows us to mold and manipulate the environment....to mold and manipulate emotional and psychological and "instinctive" states; to do this in a way that is literally unthinkable for monkeys and whales and lions and dogs and cats and muskrats and squirrels. But that is only a very, very recent phenomenon in the overall evolutionary scheme of things. We still share in common with all other mammals a reptilian brain that acts more as the engine rather than the caboose of the motivational train.
Or just look at human history [even recent history] and ask yourself: is this a species propelled by Broca's brain in conducting social, political and economic relationship? or by more deepseated, primordial parts instead?
Obviously a fluid [and combustible] combination of both. And it varies considerably from culture to culture and communtity to community and person to person. But how far do you suppose any of us can really go with our logical assessments before we bump into the parts that have less discerning or discriminating or sober and sound agendas?