The sensitive period for learning language may be specific to certain aspects of language, but it has analogues in developmental phenomena throughout the animal kingdom, most notably bird song.
Among the branches of philosophy, I had, at an earlier period, given some attention to logic, and among those of the mathematics to geometrical analysis and algebra â three arts or sciences which ought, as I conceived, to contribute something to my design.
As to past Experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist.
I shall add, for a further confirmation of the foregoing theory, that, as this operation of the mind, by which we infer like effects from like causes, and vice versa, is so essential to the subsistence of all human creatures, it is not probable, that it could be trusted to the fallacious deductions of our reason, which is slow in its operations; appears not, in any degree, during the first years of infancy; and at best is, in every age and period of human life, extremely liable to error and mistake.
There are many philosophers who, after an exact scrutiny of all the phenomena of nature, conclude, that the WHOLE, considered as one system, is, in every period of its existence, ordered with perfect benevolence; and that the utmost possible happiness will, in the end, result to all created beings, without any mixture of positive or absolute ill or misery.
For even granting this to be the case (which can seldom be supposed), the very continuation and exertion of a like energy (for it is impossible it can be absolutely the same), I say, this exertion of a like energy, in a different period of space and time, is a very arbitrary supposition, and what there cannot possibly be any traces of it in the effects, from which all our knowledge of the cause is originally derived.
It is interesting to observe that this assumption persisted even through the period of German romanticism, which was, of course, much preoccupied with the diversity of cultures and with the many rich possibilities for human intellectual development.
And in general it seems appropriate to construe both the psychology and the linguistics of the romantic period as in large part a natural outgrowth of rationalist conceptions.
Whitney’s claims regarding the diversity of languages are reiterated throughout the modern period; Martin Joos, for example, is simply expressing the conventional wisdom when he takes the basic conclusion of modern anthropological linguistics to be that “languages can differ without limit as to either extent or direction.
If the preliminary results of Mehler and Bever are correct, then it would follow that the “final stage,” in which conservation is properly understood, was already realised at a very early period of development.