Lennstar argues that human bodies cannot be treated as property. This is contradicted by the institution of slavery. Slavery is the ownership of people's bodies, i.e. people's bodies are property that can be transferred. If someone can "own" another person as property, why can't someone "own" themselves? I don't get it...

Semantic fields cannot be isolated and treated as monolithic truths, they tend to overlap with or be overlapped by facts they cannot adequately encompass in and of themselves. It may be worth pointing out 'property' has at least 10 definitions; one of which is "a person, especially one under contract in entertainment or sports, regarding as having commercial value". This is why when arguing with syllogisms based on semantic fields we frequently use the qualifiers of 'some' not just 'all'. Here the qualifier of 'all' is the root of your disagreement with lennstar.

Let's write it out then: (A) Property is (B) transferrable. Your (C)body is not (B) transferrable. Thus your (C) body is not (A) property. Simplified: A=B, C≠B therefore A≠C : I believe this is a syllogistic fallacy known as the fallacy of the undistributed middle. If am in error, please correct me!