Harnish, "Implicature and logical form", in Kasher, Implicature

R. M. Harnish, in "Logical Form and Implicature", in Kasher, "Implicature", investigates
certain types of implications (for example, "Russell and
Whitehead wrote Principia" implies they wrote it
together). He claims that these kinds of implications
are not entailments, and uses a modified version of the
Gricean theory to explain them. According to Harnish, we
should distinguish between direct and indirect
conversational implicatures. Indirect implicatures
require a maxim to be flouted, direct implicatures
require that the (more important) maxims are intended to
be observed. The implications Harnish investigates are
claimed to be of the second type.