Conception is the consensus. If a baby is lost due to miscarriage, we pray that the soul finds its way to heaven. I claim not to be an expert when it comes to souls and aether, but I know enough that they exist, that they are absolutely needed for life.

Though how they manifest can be different depending on the being, but every being born that lives through such has one.

[Which... explains some views on still births, for example. Also, seeing as he's lugging around the physical manifestation of a soul, or half of one, he's very reasonably convinced in his argument for souls. Especially when this one has existed for a veritable eternity.]