This paper defends the Blockian Proviso against its critics, Kinsella in particular, and interprets it as a law of non-contradiction in the theory of just property rights. I demonstrate that one may not lawfully appropriate in such a way as to forestall others from appropriating an unowned land because such appropriation would result in conflict-generating norms, and conflict-generating norms are not rationally justifiable and just norms. The Blockian Proviso, which precludes forestalling, operates therefore at the level of original appropriation and (...) determines, according to the homestead principle of justice in first acquisition, what may and what may not be lawfully appropriated. Hence, the Blockian Proviso is not an add-on to the homestead principle but part and parcel thereof. (shrink)