That thought cannot exist without speech, is a truth generally admitted. The negations of this thesis are all founded on equivocations and errors.
The first of the equivocations is that of those who observe that one can likewise think with geometrical figures, algebraical numbers, ideographic signs, without any word, even pronounced silently and almost insensibly within one; that there are languages in which the word, the phonetic sign, expresses nothing, unless the written sign also be examined, and so on. But when we said "speak," we intended to employ a synecdoche, by which was to be understood "expression" in general, for we have already remarked that expression is not only so-called verbal expression. It may or may not be true that certain concepts may be thought without phonetic manifestations. But the very examples adduced to show this also prove that those concepts never exist without expressions.
Others point out that animals, or certain animals, think and reason without speaking. Now as to how, whether, and what animals think, whether they be rudimentary men, like savages who refuse to be civilized, rather than physiological machines, as the old spiritualists maintained, are questions that do not concern us here. When the philosopher talks of animal, brutal, impulsive, instinctive nature and the like, he does not base himself on such conjectures as to dogs or cats, lions or ants; but upon observations of what is called animal and brutal in man: of the animal side or basis of what we feel in ourselves. If individual animals, dogs or cats, lions or ants, possess something of the activity of man, so much the better, or so much the worse, for them. This means that in respect to them also we must talk, not of "nature" as a whole, but of its animal basis, as being perhaps larger and stronger in them than the animal basis of man. And if we suppose that animals think and form concepts, what kind of conjecture would justify the assertion that they do so without corresponding expressions? Analogy with man, knowledge of the spirit, human psychology, the instrument of all our conjectures as to animal psychology, would constrain us on the contrary to suppose that if they think in any way, they also somehow speak.