 I hope I have no where said, there is no such sort of creatures in nature as birds; if I have, it is both contrary to the truth and to my opinion. This I do say, that there are real constitutions in things from whence these simple ideas flow, which we observ’d combined in them. And this I farther say, that there are real distinctions and differences in those real constitutions one from another; whereby they are distinguished one from another, whether we think of them or name them or no. (Locke, Works, vol. IV, pp. 305–6)