He saw clearly that in bed he would never think things through to a rational conclusion. He remembered how even in the past he had often felt some kind of slight pain […] which, when he got up, turned out to be purely imaginary. (1.8)
Here we see Kafka's irony coming out. Gregor thinks he can only think clearly if he's out of bed. But Gregor is thinking this thought while he's in bed. Thus how can we believe Gregor that he'll think more clearly if he's out of bed? It's roughly the same problem as the liar from Crete who says "I lie." Is the liar lying when he says he's lying?
Gregor immediately fell down with a little cry onto his numerous little legs. This had hardly happened when for the first time that morning he had a feeling of physical well-being […] and he already believed that final recovery from all his suffering was imminent. (1.29)
That fellow Descartes we mentioned in Quote #1 also formulated a philosophical problem we know as the mind-body split: can the mind exist independently of the body? What's interesting here is that Gregor's physical experiences affect his mental life to the point that they can create a belief (the belief that "final recovery from all his suffering" is possible).
He again told himself that it was impossible for him to stay in bed and that the most rational thing was to make any sacrifice for even the smallest hope of freeing himself from the bed. But at the same time he did not forget to remind himself occasionally that thinking things over calmly – indeed, as calmly as possible – was much better than jumping to desperate decisions. (1.12)
Yet another wonderful instance of irony. Gregor thinks that the most rational thing to do is to get out of bed at all costs. But then he also thinks that "thinking things over calmly" is better than "jumping to desperate decisions." Yet that's exactly what he does when he goes into a panic at the sound of his supervisor's voice – he just flips out of bed. This is not a rational act, but an act of thoughtless desperation.
When Gregor Samsa woke up one morning from unsettling dreams [...] "What's happened to me?" he thought. It was no dream (1.1-2)
The philosopher René Descartes once wondered how it was possible to distinguish between dream and reality. His answer? God wouldn't be so cruel (read more). Kafka plays on the Cartesian reference here. How does Gregor know it's not just a dream? Isn't being a bug sure evidence that he's stuck in a dream? Would God be so cruel?
[T]ormented by self-reproaches and worry, he began to crawl […] and finally in desperation, as the whole room was beginning to spin, fell down onto the middle of the big table. (2.26)
In this passage, Gregor seems on the point of losing his mind – and losing control over his body. Throughout the story, there are numerous instances where Gregor falls to the floor when he loses his mind (see our discussion of Quote #8 under "Man and the Natural World"). These instances look ahead to Gregor's last moments, where he literally drops dead (see Quote #9).
When he heard his mother's words, Gregor realized that the monotony of family life, combined with the fact that not a soul had addressed a word to him, must have addled his brain in the course of the past two months, for he could not explain to himself in any other way how in all seriousness he could have been anxious to have his room cleared out […] Even now he had been on the verge of forgetting, and only his mother's voice, which he had not heard for so long, had shaken him up […] he could not do without the beneficial influence of the furniture on his state of mind. (2.21)
This passage reflects some of the contrary impulses that push and pull within Gregor. On the one hand, family life is blamed for "addl[ing]" his brain, confusing him to the point that he's willing to give in to his insect impulses. Gregor doesn't admit the possibility that he's becoming habituated to the life of an insect on his own, just by the fact of living in an insect body. On the other hand, his mother's voice calls him back into a revived sense of his humanity. But then the passage ends with an odd comment about the "beneficial influence of the furniture" – as if Gregor had suddenly become an expert on feng shui.
But the very next [apple] that came flying after it literally forced its way into Gregor's back; Gregor tried to drag himself away, as if the startling, unbelievable pain might disappear with a change of place; but he felt nailed to the spot and stretched out his body in a complete confusion of all his senses. (2.28)
This passage is another instance where Gregor's physical experience affects his mental life. His physical pain here paralyzes his faculty for both thought and action.
"If he could understand us," his father repeated and by closing his eyes, absorbed the impossibility of the idea, "then maybe we could come to an agreement with him." (3.24)
While the story is ruthlessly ironic toward Gregor, it's equally ruthless in its treatment of the other characters. This passage beautifully stages Mr. Samsa's unwillingness to believe that Gregor can understand the others. There's no evidence to support Mr. Samsa's claim; he just assumes that it's impossible. The fact that this belief is irrational is emphasized by the fact that the story tells us he "absorbed" the idea – he didn't arrive at the conclusion through a process of logical thought.
He remained in this state of empty and peaceful reflection until the tower clock struck three in the morning. He still saw that outside the window everything was beginning to grow light. Then, without his consent, his head sank down to the floor, and from his nostrils streamed his last weak breath. (3.29)
Poor Gregor! Shouldn't the fact that he's a bug should make the death less pathetic? But this passage is still incredibly sad. As in earlier passages (see Quote #6), when Gregor loses consciousness, he also loses control over his body, as the phrase "without his consent" emphasizes. But the fact that his last moments are spent in "empty and peaceful reflection," instead of anxiety, anger, or frustration, makes his situation almost enviable. If being an animal has resulted in the deterioration of his capacity for reason in any way, it also took away his capacity to torment himself with his own thoughts.
[The cleaning woman] thought that [Gregor] was deliberately lying motionless, pretending that his feelings were hurt; she credited him with unlimited intelligence. (3.30)
As you have probably noticed by now, Kafka's story is quite ironic about man's vaunted capacity for reason. It stages scene after scene where characters are unaware of the absurdity of their claims – not just Gregor, but the other characters as well. The irony is underscored here by the fact that the cleaning woman of all people is willing to ascribe to Gregor "unlimited intelligence." As far as we know, the cleaning woman's only area of expertise is, well, trash, refuse, garbage. Is the story suggesting that intelligence is, well, a waste?