In his New Essays On Human Understanding, Leibniz’s mouthpiece Theophilius makes somewhat frequent reference to what he calls “blind thought” (more accurately, according to the notes in the Bennett/Remnant translation, Leibniz’s typical usage is the French phrase pensées sourdes, which would be “deaf” or “muffled” thoughts, but he equates this with the latin phrase cogitationes caecae and so Bennett/Remnant opted for translating this as “blind thought”).
This category of thought occurs when the mind manipulates symbols without having the ideas signified by those symbols present to our minds. Leibniz’s helpful illustration (p. 186 in the Bennett/Remnant translation) is the case of “those who calculate algebraically with only intermittent attention to the geometrical figures which are being dealt with.” Leibniz goes on to say:
Words ordinarily do the same thing, in this respect, as do the symbols of arithmetic and algebra. We often reason in words, with the object itself virtually absent from our mind. But this sort of knowledge cannot influence us—something livelier is needed if we are to be moved. Yet this is how people usually think about God, virtue, happiness; they speak and reason without explicit ideas—it is not that they cannot have the ideas, for they are there in their minds, but that they do not take the trouble to carry the analysis through. (p. 186)
This discussion, in which Leibniz first introduces blind thought, occurs in the midst of Leibniz’s commentary on Locke’s views on power and freedom. Specifically, it appears that Leibniz introduces the notion in response to Locke’s view that the main determinant of the will is not the prospect of a greater good, but instead, some strong present unease. After explaining blind thought, Leibniz explains that “if we prefer the worse it is because we have a sense of the good it contains, but not of the evil it contains or of the good which exists on the opposite side.”
As suggested by the initial illustration of algebraic reasoning, Leibniz’s stance on blind thought is not that it is always problematic. In a later discussion, relating to the purpose and origins of language, Leibniz suggests that blind thought can be of great utility:
I believe that without the desire to make ourselves understood we would indeed never have created language. Once created, however, it also enables man to reason to himself, both because words provide the means for remembering abstract thoughts and because of the usefulness of symbols and blind thoughts in reasoning, since it would take too long to lay everything out and always replace terms by definitions.” (p. 275)
This category of blind thought is in some ways very much like the initial case Berkeley uses to challenge the Lockean thought that words are significant only insofar as they signify ideas (I should note that I am using the term “Lockean” because both Berkeley and Leibniz appear to conceive of themselves as rejecting something that Locke affirmed—it may be that, on careful reading, we shouldn’t think that Locke’s views rule out the sort of cases Leibniz is enumerating here). Berkeley’s case example is the use of chips or counters while playing card games. The counters stand for pounds and shillings even if we do not keep the ideas of pounds and shillings in mind while playing. Further, it is easy to read a similar view about the utility of reasoning with symbols instead of ideas in Hume’s discussion of role of numerals in our complex mathematical reasonings. While it is worth noting that Berkeley goes on to offer a much more radical rejection of the necessity of ideational signification for meaningful terms (as I have discussed before), it is quite interesting that Leibniz, Berkeley, and Hume all seems to recognize some potential for value (and some potential for error) in this sort of blind thought.
This also means that they each face the challenge of offering a basis for separating the useful/productive cases of blind thought from the harmful/error-producing cases. Given the context of the discussion in the New Essays, it is plausible to think that the problem cases (for Leibniz) are those in which the presence of the ideas signified by the terms would have motivational relevance. I am not, however, a Leibniz scholar, and don’t know if there are other texts which speak to his views on blind thought that would be worth investigating alongside the passages from the New Essays.