 This prohibition on any “explanatory flow” (Bennett 1984) between mental and physical realms is the core meaning of Spinoza’s attribute barrier doctrine. Given Spinoza’s commitment to universal intelligibility (see 1.1), derivatively, the impossibility of conceptual relations between minds and bodies implies the impossibility of causal relations between them (E3p2s). For if something is in principle unintelligible (as any purported causal relation between minds and bodies would be), it is also metaphysically impossible. This prohibition on causal interactions between mental and physical things is the key secondary meaning of the barrier doctrine.