This is my second ever question. I am reading Schopenhauer's "On the fourfold root of the principle of sufficient reason". I am a beginner to philosophy, and received a very helpful reply to my previous question (thanks Victoria ;-) ). I have what I'm afraid is a somewhat dumber one than the last as it's really more of a niggle and a display of my lack of confidence with this material - but I'm the sort that can't leave a niggle alone until I'm satisfied, so here I am.
My issue is with the passage below from the chapter "First Class of Objects for the Subject". Schopenhauer is mostly very clear in this book, but at certain points he says something that sounds wrong to me, such as here when he discusses the Understanding in relation to our sense of sight. Again the italicised passage is the part I am unsure of:
"As each point of the visible object sends sends forth its rays towards all sides in a rectilinear direction, the rays from its upper extremity cross those from its lower extremity in the narrow aperture of the pupil, by which the former impinge upon the bottom, the latter upon the top, those projected from the right side upon the left, and vice versa. The refracting apparatus of the eye, which consists of the humor aqueus, lens, et corpus vitreum, only serves to concentrate the rays of light proceeding from the object, so as to find room for them on the small space of the retina. Now, if seeing consisted in mere sensation, we should perceive the impression of the object turned upside down, because we receive it thus ; but in that case we should perceive it as something within our eye, for we should stop short at the sensation. In reality, however, the Understanding steps in at once with its causal law, and as it has received from sensation the datum of the direction in which the ray impinged upon the retina, it pursues that direction retrogressively up to the cause on both lines ; so that this time the crossing takes place in the opposite direction, and the cause presents itself upright as an external object in Space, i.e. in the position in which it originally sent forth its rays, not that in which they reached the retina (see fig. 1).
The purely intellectual nature of this process, to the exclusion of all other, more especially of physiological, explanations, may also be confirmed by the fact, that if we put our heads between our legs, or lie down on a hill head downwards, we nevertheless see objects in their right position, and not upside down; although the portion of the retina which is usually met by the lower part of the object is then met by the upper: in fact, everything is topsy turvy excepting the Understanding.
...now I may be overcomplicating things due to not being confident at visualising and spacial awareness: but what on earth does he mean by saying we don't see objects upside down when we put our heads upside down? Does he just mean that the Understanding is not fooled, that we still know which way is up despite the reversed image? Because obviously we DO see objects upside down when we look at them...upside down. I feel I'm missing something so obvious. Can anyone reassure me?