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One ought to love one's neighbor.
Literal
Neighbor [topic-は] should-love [べき-copula-である].
~べきである is the formal/written version of ~べきだ — a formal obligation 'should, ought.' 愛すべき ('worthy of love, ought to be loved') is literally the passive-volitional-like reading of 愛す + べき. Formal/biblical register.