In his Critique of Pure Reason, Kant posits two seemingly contradictory claims:

  1. The nature of Things in Themselves, as they exist apart from the phenomenal world, are unknowable.
  2. Time and Space do not exist as Things in Themselves but are only applied to our perception of the phenomenal world.

Ergo, if ALL Things in Themselves are unknowable, we can't also know that space and time do not exist as Things in Themselves, surely?

How do Kantian's respond to this argument, known as the "Neglected Alternative" and what is the present state of the debate?

  • Things in themselves are a logical a priori since something must have affected our senses which then leads to the manifold of intuition and via further processing to intuitions and representations proper. It is a placeholder for the ultimate cause of that which we come to experience, for the matter of intuitions. Space and time, by contrast, are mere forms of our perception, ie. shown to only be our modus operandi and not inherently tied to objects/things in any sense, even less something received via our senses, thus do not have the corresponding cause. What is unclear about that? – Philip Klöcking May 11 '20 at 13:00
  • @PhilipKlöcking because if the features of the noumenal world are unknowable, then how can we assume space and time are merely forms are perception? – Charlie May 11 '20 at 13:03
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    He dedicates a book (his dissertation) and a whole chapter of CPR to show that they are mere forms of intuition. This does not exclude that there may be something corresponding to space and time in the noumenal world, but they are not things in themselves nor, as mere forms, necessitate to posit a thing in itself. By Kant, it is idle speculation to even consider the noumenal as containing particular "things" since things are our way to represent objects and there is no reason to believe the noumenal is in any way like anything we can even imagine. – Philip Klöcking May 11 '20 at 13:09
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    The thing is: Space and time we can know, since they are part of our understanding, while the noumenal is beyond it. That we can know about space and time already tells us that they are not noumenal. That's the whole point of transcendental philosophy. – Philip Klöcking May 11 '20 at 13:11
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    A standard response is that it is neither neglected nor an alternative, it is a category mistake that Kant considered and dismissed. To say that our forms of intuition are "really" there requires pretending that we can apprehend things in themselves, a purely discursive concept, via intuition. It can not be sensible intuition, as that is limited and peculiar to us, so it would have to be the (divine) intellectual intuition of things' creator. And that we lack, says Kant, see e.g. Bonaccini. – Conifold May 11 '20 at 13:11

Short answer : a thing in itself ( a monadic substance) does not depend on its ( spatio-temporal) relational properties to exist ( to the contrary, relations depends on substances; relations are accidents ) ; but the things we experience need spatio-temporal properties to exist; so the things we experience are not things in themselves.

When Kant thinks of " things in themselves" what he has in mind is leibnizian monadic substances.

According to Leibniz, space and time are ordering relations applying to monads.

if it is the case, these spatio-temporal relations depend on the existence of these monads.

Kant's argument could be reconstructed like this :

(1) If space and time were relational properties of things in themselves, then space and time could not be conceivable in case the things that are in space and in time were suppressed. For relations having an absolute extra-mental being are not conceivable without their " in themselves" relata.

(2) But, in fact, we can conceive or represent ourselves space and time, spatio-temporal relations, without anything being related by them. I can represent myself an empty space and an empty time.

(3) So, the things that are related by spatio-temporal relations are not things in themselves ; they depend on spatio-temporal relations to " exist" ( and not the other way round) ; they are not monadic substances, but " phaenomena" ( mind dependent entities).

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