Schopenhauer derived body-as-will doctrine from Kant’s moral law view.

Evidence

It follows that, for Kant, volitions must belong to the real world, not to the world of phenomena. The phenomenon corresponding to a volition is a bodily movement; that is why, according to Schopenhauer, the body is the appearance of which will is the reality. — Bertrand Russell, History of Western Philosophy, 2737

Categorization

• XREF: Shows a logical derivation chain running from Kant through to Schopenhauer — relevant to tracing idea lineages. — connects to: Will-primacy doctrine, not pessimism, is Schopenhauer’s most influential legacy. — Bertrand Russell, History of Western Philosophy, 2752; Living morally reveals deeper truths about life and existence. — Leo Tolstoy, A Confession, 155; Life’s essence may reduce to the will to exist. — Leo Tolstoy, A Confession, 79 (+1 more)
• corroborated: 2 independent passages support this claim — Bertrand Russell, History of Western Philosophy, 2737

Broader Connections

• Life’s essence may reduce to the will to exist. #expands (0.80) — Leo Tolstoy, A Confession, 79
• Living morally reveals deeper truths about life and existence. #corroborates (0.75) — Leo Tolstoy, A Confession, 155
• Moral rules cannot be created or altered by deliberate human enactment #corroborates (0.60) — HLA Hart, The Concept of Law (Clarendon Law…, 443