“XIII. The object of the idea constituting the human mind is the body, in other words a certain mode of extension which actually exists, and nothing else.

Proof—If indeed the body were not the object of the human mind, the ideas of the modifications of the body would not be in God (II. ix. Cor.) in virtue of his constituting our mind, but in virtue of his constituting the mind of something else; that is (II. xi. Cor.) the ideas of the modifications of the body would not be in our mind: now (by II. Ax. iv.) we do possess the idea of the modifications of the body. Therefore the object of the idea constituting the human mind is the body, and the body as it actually exists (II. xi.). Further, if there were any other object of the idea constituting the mind besides body, then, as nothing can exist from which some effect does not follow (I. xxxvi.) there would necessarily have to be in our mind an idea, which would be the effect of that other object (II. xi.); but (I. Ax. v.) there is no such idea. Wherefore the object of our mind is the body as it exists, and nothing else. Q.E.D.”

Why does Spinoza want to go over the trouble of saying “if we do have an idea of something other than our bodies, then that idea of something else must cause an effect. But we don’t have an idea of that effect. Hence we don’t have an idea of the cause/something other than our body.”

Can’t Spinoza just say “we don’t have an idea of things other than our body” from the outset? Why does he bring into here a discussion of cause and effect?

  • The only reason I think why Spinoza would include a discussion of cause and effect, is this: some people may think they have an idea of a tomato. But in reality, the idea of the tomato is just an idea of themselves. If a person were to leave the room where the tomato is in, and the tomato gets cut, the person wouldn’t know the tomato has been cut. Therefore that person doesn’t have a true idea of the tomato. Here, the tomato is the cause, the cut is the effect. The person doesn’t know the effect, so he doesn’t truly know the cause. He only knows himself as perceiving that tomato Aug 6, 2020 at 2:01
  • Spinoza's core recognition of the nature of human knowledge has as it's origin his recognition that for each effect there must necessarily be a cause. Therefore if we can understand that cause then we will have obtained knowledge about something's origin. Put enough of these individual understandings into a 'chain' of interconnected ideas and you will soon have an effective mental l'ibrary'. From that you can create various disciplines like physics, chemistry, psychology, etc. As for your 'tomato', Spinoza was not interested in artificially contrived scenarios, but real time objects in act.
    – user37981
    Aug 7, 2020 at 4:11
  • @CharlesMSaunders , hrm, but here in 2P13, Spinoza is arguing that we don’t know anything besides our own body. And he argues for it like this: “if we did have an idea of something other than our body, we would know an effect of that idea. But we don’t know that effect, therefore we don’t know the cause, or an idea other than our body.“ Isn’t this just as good as saying “we don’t have an idea of something other than our body, because we have no ideas of things other than our body?” Aug 7, 2020 at 4:30
  • What he is saying is that due to the firewall between the Attributes of Thought and Extension, the mind can have no direct experience of the external world, except through the mediation of the body. Check Prop 12, Part 3 and the proof and explanation. The relation between the body and the mind is convey through the 'conatus', The body gathers 'impressions' from objects and people, these transform into emotional stimulus which are conveyed to the brain and thence become ideas in the mind. If you wish, see my, 'Spinoza's Strange Symbiosis- Where Emotion and Thought Conjoin', on academia.edu.
    – user37981
    Aug 8, 2020 at 14:23

1 Answer 1


I’ve figured it out. I read ahead a little, and Spinoza states later on that the mind does not know the body in itself, but only the modes of the body. This also means that if a foreign object is part of the mind, we wouldn’t know that foreign object in itself. However, we would know the modes of that foreign object, or the effects of that foreign object. But all the stuff we know have to do with modes of our body, therefore there is no idea of a mode of a foreign object. Hence our mind does not include a foreign object.

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