In Jainism, the answer is very clear and very precise:

1. When does the soul enter?

- The soul enters at the very first moment of conception – when sperm and ovum unite and a new body (zygote) is formed. - That first one-sensed embryo (ekendriya sharir) is already a jiva (living being). There is no “empty body” phase and then later soul-entry. From the first moment, there is soul + body together.

1. How does the soul enter? (According to Jain philosophy)

In simple steps:

- At the time of death of the previous life, the next birth (gati, yoni, etc.) is fixed according to that soul’s karmas. - Death and next birth are instantaneous: the soul leaves the old body and in the very next samaya (smallest time unit) it takes a new body appropriate to its karma. - If the next birth is human/animal, that soul is automatically drawn (by its own karma) to a suitable womb where sperm and ovum have just united. - There is no physical “travelling” like air entering a balloon; rather, the soul (an eternal, non-material substance) simply becomes associated with that just-formed body along with its subtle karmic body (karmana śarīra) and electric body (taijasa śarīra). - From that first instant, all growth of the embryo, fetus, child, etc. is growth of a living being (jiva bound with matter), not dead matter.

1. Is this different for Tirthankars?

- The basic rule is the same for every soul: birth begins when the soul enters the new body at conception (this is celebrated as Garbha Kalyanak in the case of Tirthankars). - For Tirthankars, scriptures describe this event in more detail (descent from heaven, auspicious dreams of the mother, etc.), but the principle is identical: soul + first embryo = start of that life.

1. Important points from Jain view

- The soul is beginningless; only a new bodily life begins at conception. - There is no gap of time between leaving one body and entering another. - Every embryo that forms and is “taken up” by a soul is a full jiva from that first moment, deserving of ahiṁsā (non-violence).

You can read a simple explanation connected to this idea of life beginning at conception (Garbha Kalyanak) here: