 Nevertheless, the notion that reasoning is computation has been referred back to more than once. Leibniz explicitly endorsed and developed it in one early work: “Thomas Hobbes, everywhere a profound examiner of principles, rightly stated that everything done by our mind is a computation, by which is to be understood either the addition of a sum or the subtraction of a difference … So just as there are two primary signs of algebra and analytics, + and −, in the same way there are as it were two copulas, ‘is’ and ‘is not’” (Leibniz 1666, 3). And the idea appears to have continued to hold some appeal for him. Thus for example Leibniz’s numerical characteristic (Leibniz 1679) attempts in another way to use the language of addition and subtraction to explain aspects of reasoning.