  As our ability to undertake more powerful Searches for Extraterrestrial
Intelligence (SETI) grows, so does interest in the more controversial endeavour
of Messaging Extraterrestrial Intelligence (METI). METI proponents point to the
SETI Paradox - if all civilisations refrain from METI then SETI is futile. I
introduce Mutual Detectability as a game-theoretic strategy aimed at increasing
the success potential of targeted SETI. Mutual detectability is embodied by
four laws: mutuality, symmetry, opportunity and superiority. These laws
establish how SETI participants can engage each other using game theory
principles applied to mutual evidence of mutual existence. The law of
superiority establishes an "onus to transmit" on the party whom both SETI
participants can judge to have better quality evidence, or common denominator
information (CDI), thus avoiding the SETI Paradox. I argue that transiting
exoplanets within the Earth Transit Zone form a target subset that satisfies
mutual detectability requirements. I identify the intrinsic time-integrated
transit signal strength as suitable CDI. Civilisations on habitable-zone
planets of radius $R_{\rm p}/R_{\oplus} \lesssim (L_*/L_{\odot})^{-1/7}$ have
superior CDI on us, so have game-theory incentive (onus) to transmit. Whilst
this implies that the onus to transmit falls on us for habitable planets around
$L_* > L_{\odot}$ stars, considerations of relative stellar frequency,
main-sequence lifetime and planet occurrence mean such systems are likely a
small minority. Surveys of the Earth Transit Zone for Earth-analogue transits
around sub-solar luminosity hosts, followed up by targeted SETI monitoring of
them, represent an efficient strategy compliant with mutual detectability. A
choice to remain silent, by not engaging in METI towards such systems, does not
in this case fuel concerns of a SETI Paradox.
