What do you mean by "without a brain"?

There are multiple examples in the literature of people leading perfectly ordinary lives whilst unknowingly having no more than 5% of the typical amount of brain matter (typically because of hydrocephalus). For example, https://www.science.org/doi/10.1126/science.7434023 from 1980.

They mean stuff like https://en.wikipedia.org/wiki/Anencephaly.

The brain is indeed incredibly resilient - some kids with serious epilepsy get an entire hemisphere taken out - but which 5% you're left with matters enormously.

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