Suppose that a constructed system of deontic norms is still under construction: there are various good candidate actions for being impermissible that we have not yet settled on as impermissible (perhaps this matter is in dispute and that there are such disputes is part of how we get impermissibility norms that are sensitive to relevant agents and their aims). We might, and standard deontic logical systems do, stipulate that, in such cases, permissibility is the default. So such candidates for being impermissible would be permissible (until made impermissible). More generally, as a conceptual matter, permissibility is simply non-impermissibility. What is being demarcated is something like the distinction between the social or personal normative standards that constitutes impermissibility and the sheer absence of such. And there are probably good reasons for conceptualizing things this way, having to do with how deontic rule-governed systems function and reliably achieve their aims.
But we can divide up this same normative territory differently with our concepts (the phone rings for Ayn Rand and Derrida and various “deconstructionists”). If, for whatever reason, we were equally concerned with active allowing (in attitude and action) and active disallowing (in attitude and action) — but not so concerned with null or indifferent or ambiguous reaction — then we would sensibly use a threefold permissible*/ impermissible*/ neither distinction for the norms governing these three types of responses (and I’m sure a clever logician could come up with a three-value deontic logical system to formalize how the inferences would go).
Because we do not think of deontic matters in this way, here is a property that is somewhat apt to get ignored: actions that neither call for being actively allowed nor call for being actively disallowed (but instead call for no response at all or no response of the relevant kind). We might, in a given context, erroneously slip into thinking that permissibility is a function of norms that call for something like active allowing, accepting, etc.
This is relevant to how consent and somewhat-consent-y non-consent actions and relationships might generate permission. Because permissibility is simply non-impermissibility it is always open, at the most fundamental level, that the mere failure of disallowing or resistance meeting the relevant standard suffices for generating permission. E.g., this pattern: X does A to Y and Y does not resist (but the non-resistance is not due to suspect/nullifying reasons or circumstances such as coercion, gross lack of information or deception or brainwashing, normal adult competence, etc.).
(I take the ‘does not resist’ condition above from Bernard Williams’ essay “Realism and Moralism in Political Theory” and this sort of formula fits well with Williams somewhat-skeptical or less-than-full-throated defense of liberalism. But we could just as well say ‘accept’ or ‘allow’ as ‘not resist’ as long as we take the relevant sort of accepting or allowing to include merely the absence of rejection, resistance, etc.)
Though local liberal reasons grounded in the value of individual autonomy or some such might well carry the day against, we should be open to the idea that actions or relationships that fit something like the above pattern could do the same work as corresponding actual, explicit consent.
(It is worth contrasting my suggested pattern for getting consent-like normative work done — and the reasons I give to support it — to David Estlund’s idea of hypothetical, normative consent doing the same work as consent in the theory of political legitimacy — and the (weak, I think) reasons he gives to support that. David Archard, in his book SEXUAL CONSENT (ch. 1), uses the label ‘ersatz consent’ to refer to various ways in which it has been proposed that the normative work of consent, in various contexts, might be done in somewhat-consent-like ways but without literal consent (not even implied or tacit). So, in these terms, what I have suggested here is a pattern for a kind of ersatz consent that is a function of some conceptually and metaphysically deep facts about what permissibility is. Though I tried to avoid it, I suspect there are some suppressed assumptions in the above treatment, to the effect that permissibility is essentially, at least in part, a function of reasons favoring non-forbidding or permitting, not (or not merely) reasons to do or not to do the permitted thing. However, the point here is meant to be supported independently of any such (controversial) assumptions about what permissibility comes to.)