What is the “taam” (or lomdus) why psik reisha is ossur?
Answer:
From memory, the Rashba implies that although the person does not have concerte intention for the melachah, the fact that the melachah will inevitably happen is sufficient for us to see it as though he has intent (the wording is that “it is as if he intended it”). Another approach is that the intent is “included” in the action.
An important point to note is that the fact that pesik reisha is forbidden is not obvious. The halachah of Rav Shimon is ruled in many places, and is found in several Tanaic sources, whereas the concept of pesik reisha is “only” a meimra of Abaye, an Amora.
Yet, nobody explicitly disputes the principle, and the idea of pesik reisha is of course ruled in halachah.
Apologies for the brevity of the answer. Many acharonim discuss the issue, and the learned questioner will surely supplement sources.
