4 ideas
Pierre Saint-Menoux uses a time-travel substance to go back to 1793, during the siege of Toulon, hoping to change history by eliminating the young Bonaparte. In the confusion he kills a soldier who turns out to be his own ancestor, and he immediately vanishes from existence. This episode is the earliest widely cited fictional statement of the grandfather paradox.
If a traveller acts in the past to prevent his own lineage, he removes the cause of his own existence, and with it the cause of the act itself. The contradiction arises because the actor is treated as independent of the chain he is editing, when he is actually one of its downstream products.
Any intervention in history is made by someone the unedited history produced. Removing one node, such as a single dangerous man, also severs countless unseen threads, and the intervener's own existence may be among them. Reforming the past therefore carries a risk that grows with how deeply the reformer is embedded in what he changes.
In the far future, humanity has split into distinct species, each biologically shaped for a single task, so that society works like one body and individuals work like its organs. Seen this way, collective efficiency pushed to its limit does not free people. It removes individuality, choice and purpose beyond function.