mixed_biscuits
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This argument is flawed because if you followed it to a logical conclusion you'd be getting into "every sperm is sacred" territory, and everybody would be having as many children as possible, because otherwise you are not allowing people to exist. Which is absurd.
If you leave a sperm on its own for a year, you won't find some gurgling babe there one morning. I'm referring to the fertilised egg and its (barring natural disaster) inevitable progression to adulthood.
'consciousness in the womb debate'
Well, it's an interesting debate but it's ultimately undecidable, so of little use in the matter at hand. The inner life of a foetus is inaccessible to us, just as the inner world of the bat is (per Thomas Nagel).
Furthermore, 'feeling pain' must not the sole criterion of a 'valid existence,' for obvious reasons.