Immanuel doesn't pun. He Kant.

Even Oscar Wilde appreciated a good Kant pun. Kant takes a break from some heavy duty postulating to bring you: The Refutation of Idealism! 

Kant begins his refutation of idealism by clearly defining what it is he is arguing against. He terms it "material idealism" and divides it into two categories, with each holding that the existence (or reality?) of external objects is either uncertain or flat-out impossible. These two related but differing schools of thought are attributed to Descartes and Berkeley, respectively. We are familiar with what Kant is referencing in each case: 

     - Descartes' Meditations hold that the only thing which can be empirically ascertained is the mind; "I think therefore I am" holds that the mind can only be proven to be real at the exact moment in which that thought occurs. Everything else is kind of a crapshoot. Kant terms this "problematic idealism." 

     - Next comes the "dogmatic idealism" of Berkeley, who holds space to be purely fictitious and all things thence contained in space as the same. 

Kant rather brusquely swats aside Berkeley's dogmatism with reference to the Transcendental Aesthetic. He claims that the only way one can arrive at a standpoint of dogmatic idealism is by considering space to be a property, but as we know, space is a priori, a pure intuition, and doesn't represent any property of things-in-themselves, and is also the answer to life, the universe, and everything. So now the only issue at hand becomes Descartes' doubt. Kant will attempt to prove that experience actually happens, which can be established only if our inner experience ("I am") can be possible if the external reality of the world (outer experience) is presupposed. The man managed to pack a lot into p. 326, B275. 

In a rare burst of syntactic clarity that is unlikely to be repeated, Kant's thesis is "the mere, but empirically determined, consciousness of my own existence proves the existence of objects in space outside me." He goes about supporting it thusly: 

1. I am aware of the fact that I exist in time.

2. There is something persistent in this perception. (Did someone say...THE FIRST ANALOGY? see p. 299 for more on this)

3. Whatever this persistent thing is, it cannot be internal, as my own existence is only made clear to me through this persistence in the first place.

4. Because this persistent thing is not inside of me, it must therefore be external to me (as a thing, and not just the mere representation of a thing internally).

5. In other words: the awareness of my existence is only possible because of external things.

6. Awareness of my own existence is simultaneously an awareness of the existence of externalities. 

(All of this to be found on p. 327, B276.)

Kant finishes laying the smackdown on idealism with three Notes, wherein he decides to make up for his previous clarity by ironically obfuscating the point more with what was intended to elucidate his argument.

Note 1: Idealism's major flaw is its assumption that immediate experience is confined to the internal, and from this external realities must be inferred. Rather, it is outer experience that is truly immediate to us: it is only through outer sense that we become aware of the mind's determination in time (that is, inner sense). Outer objects are a requirement for the determination of the subject. (p. 327-8, B277)

Note 2: Sensibile experience completely proves this to be the case. We only understand time via the changes of objects in space (case in point: a day is the rotation of the Earth around the Sun). The only other way we could possibly understand the persistence of the proof would be through matter, but this too is presupposed a priori, and also comes to inner sense through the existence of outer things. Consciousness is merely a representation of the subject and cannot be the persistence. (p. 328, B 278)

Note 3: Just because it must be the case that outer objects exist for there to be a determinate consciousness, this does not mean that every time we perceive an object it exists. We could be completely imagining that stuff. (p. 328-9, B 279)

This concludes our broadcast, we now return you to your regularly scheduled programming.