 This is an intriguing suggestion, but seems not to be very far developed. This addition has to follow some rules, especially in the syllogistic case. As Hobbes says, “Every man is an animal; some animal is a quadruped; therefore, some man is a quadruped” is “defective” (Hobbes 1655, 4.4). But its conclusion too involves the addition of parts of the premises. Presumably syllogistic addition, like arithmetic addition, must have its rules. And of course, Hobbes was aware of the properties of various good and bad arguments. But it’s not clear what he added to that discussion by bringing in the language of addition. Nor, indeed, is it clear what he really added to his discussion of the workings of the mind by his occasional use of such language.