“All ravens are black.”
“Non-black things are non-ravens.”?
They are, after all, logically equivalent. When we ‘count’ all non-black things and all non-ravens, what are we left with? We are left with black ravens. Thus we arrive back at ‘All ravens are black’. That is why they are equivalent. If something (or anything) is a non-black thing, it can’t be a raven. If anything is a non-raven, then (well) it can’t be a raven. A blue bird is not a raven. A banana (a non-raven) is not a raven.
All the above (talk about crows) only really makes sense in the context of natural kinds and even essences... or does the logic stand independently?
J.S. Mill believed that the world contains (natural) kinds. We don't compose these kinds. They are there in the world. They are mind-independent. This seems to go against Mill’s empiricism: the talk about mind-independence. Perhaps he wouldn’t have used the term ‘mind-independence’ in this context. Perhaps instead he would have talked about his “permanent possibility of sensation” in that if we were to observe a natural kind like a raven, we would observe x, y and z.
What binds kinds together? Their ‘common nature’.
Ravens and other species aren't the only natural kinds. Gold and water are also natural kinds. Though perhaps we should say that H2 O is a natural kind (if ‘water’ refers only to secondary properties or Locke’s ‘nominal essences’).
Yes, “we do not create the kind water simply by our classifications” (192). However, we only get to water (as it were) via our classifications. And if we only get to water via our classifications, then perhaps, in a sense, we do create that natural kind in that we only know it via our classifications and only observe it through our classifications.
Roger Scruton explains (in his excellent introductory book Modern Philosophy) what a real essence is.
Firstly, the “nominal essence” of diamonds.
We “pick out diamonds by means of their hardness and transparency” (192). Scientists have no time for these properties. They aren't only ‘interest-relative’; but also mind- and sense-dependent too. Science discovers what real essences are. Does that automatically mean that such properties won't be ‘interest-relative’ and ‘sense-dependent’? Not according to many philosophers - not least those of whom deny essence altogether.
What is the real essence of diamonds? It is carbon, “the very same thing as charcoal” (192).
How can we flesh out this essence of diamonds? We can by saying that the fact that diamonds are essentially carbon “is another instance of an a posteriori necessary truth” (192). It's necessary that diamonds are carbon. Alternatively, they are carbon at every possible at which they exist. The result of this view is that a diamond could lose its “hardness, sheen and transparency – without ceasing to be what it is” (192). Perhaps a diamond wouldn't loose its essence if it lost these ‘contingent’ properties; though would it still be ‘what it is’? It depends on what it is. And if diamonds are only their essences, then this conclusion follows by definition.
Could we still have a diamond without its sheen, transparency, hardness, etc.? Or would we just have a lump of charcoal? Certainly no layperson would recognise it as a diamond. Why is the layperson's view of what a diamond is irrelevant? Why are sheen, hardness and transparency irrelevant? Is a lump of charcoal really a lump of diamond?
Natural kinds have become very fashionable in the last, say, forty years (after a long period - in some quarters - of anti-essentialism). Why was that so? Firstly, “it suggests that science looks for necessary connections” (192). Does that mean the necessary connections between natural kinds? Does it mean that that the necessary connections between natural kinds are themselves natural kinds? Or does it mean that there are necessary connections between the properties which constitute natural kinds?
Is it really the case that “it is the task of science to discover” these necessary connections?
Essences increase the possibility of objectivity in science if we have both natural kinds and necessary connections. In addition, the interest in natural kinds and necessary connections helped “detach it from our observations and attach it to an objective order” (192). Is science really detached from our observations? We know that observations aren’t everything; though surely they account for something – perhaps much!