The external world of objective reality clearly contains objects of various kinds, shapes, colours and so forth. E.g. there are conifers (an evergreen). Many students of philosophy (following Locke) learn to distinguish between primary and secondary properties of ordinary objects. The former exist out there, in the objects themselves, e.g. their shapes (and natural kinds). The latter exist only as we perceive such objects, e.g. their colours; and so we soon reach the philosophical problem of perception: We see a tree as green, out there in the world, but the green exists only in our heads (so to speak). The modern scientific picture of the world has it that where we see the green tree are really just various biochemicals, reflecting certain electromagnetic waves toward our eyes. The green seems not really to be where we can clearly see that it is—out there, in the leaves of the tree—but to be only in the pictures of the world that our brains construct.
......Indeed, since such pictures are what we’ve been calling ‘the world’, some might think of the world as in their heads (which is one way to put the green back in the greenery). Many philosophers (following Kant) take the shapes of ordinary objects to be, not primary properties (as Locke thought), but also secondary. The world might really be composed of atoms composed of 10-dimensional strings, for example, with our brains imposing, upon the numerous sensations that come from our sensory organs, the usual 2 and 3-dimensional shapes that we see in the world around us. Indeed, since our brains may even be imposing the basic structure of a number of objects upon our sensory input, some philosophers conclude that ordinary objects just don’t exist in reality (e.g. see Jackie’s comments on my previous post, Chairs Exist). But what do we mean by ‘reality’? Surely we could only mean whatever space it is that includes the people whose language includes such expressions. So there are certainly some objects out there, i.e. other people.
......And similarly, I think, there is a sense in which the greenness that we see really is objectively out there, on the surface of such objects as leaves. That is because we learn the meaning of ‘green’ (as part of learning the concept of colour) by being shown various green objects or pictures (and red ones, etc.) and being told that they are all green (red etc.). Green is therefore something that ordinary objects can be. Basically, something is green if its surface is such that, under normal lighting conditions, it would give rise to the same sort of sensations in those looking at it as they had when they learnt the meaning of ‘green’. So when it comes to something being green—to it being true to say of it that it is green—it is irrelevant what those sensations are, whether they are the same for one person as for another (although they are probably very similar, in view of our similar physiologies); the objective greenness that we see via those subjective sensations is, by definition, less subjective than they are.
......Of course, you usually take the meaning of ‘green’ to be just such sensations as you would call ‘green’, because that is how you learnt to use that word. Indeed, we all do, and so that is also part of the meaning of ‘green’. That equivocation usually goes unnoticed—except in such philosophical contexts as the problem of perception—precisely because it is irrelevant what such sensations are (how they differ between people). And of course, the problem of perception is hardly a mistake of the order of a misperception. How else could we possibly refer to things in an external world, except via our side of our interactions with it? Perception is never a view from nowhere. Even scientific observations are careful perceptions of the external world. And when it comes to predicating existence of something, our most certain knowledge comes from some of us seeing that it is.
......Incidentally, a surprisingly good analogy for the problem of perception is a blind person, e.g. using a white stick to check her picture of where she is. Suppose her stick hits an unexpected obstacle in her path. Just from how her stick reacts to hitting it—how the other end of it feels in her hand—she can tell that it’s a bouncy, light, smoothly rolling object... presumably a child’s ball. She can knock it out of the way and carry on; but in the land of the blind, her word for such bounciness in an external object may well be the same as her word for the way her stick felt in her hand. Nevertheless, she would hardly be tempted to think of the world as full of feelings. It would be full of objects that feel one way with a stick and another to the touch, and in other ways via gloves (or other skin, hair, etc.).
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