Enchiridion XXXVI

XXXVI

As the proposition, “either it is day or it is night,” has much force in a disjunctive argument, but none at all in a conjunctive one, so, at a feast, to choose the largest share is very suitable to the bodily appetite, but utterly inconsistent with the social spirit of the entertainment. Remember, then, when you eat with another, not only the value to the body of those things which are set before you, but also the value of proper courtesy toward your host.

Courtesy towards your host is a praiseworthy thing; overall, this section is another that is similar in tone to the Havamal in several respects. 

A greedy man, if he be not mindful,
eats to his own life's hurt:
oft the belly of the fool will bring him to scorn
when he seeks the circle of the wise.

Herds know the hour of their going home
and turn them again from the grass;
but never is found a foolish man
who knows the measure of his maw.

The translation of the first line is confusing, because it gives 'it is day / it is night' as already in a disjunctive form. That makes it hard to get the point about why it lacks force as a conjunctiver argument. In symbolic logic, the two propositions look like this:

∀x(Dx∨Nx) [Disjunctive: "For any time x, either x is day or x is night"]
∀x(Dx∧Nx) [Conjunctive: "For any time x, x is day and x is night."]

The point is just that a proposition about day/night works well as a disjunctive in ordinary language, and not at all well as a conjunctive. Although, notice that the conjunctive is also* a true statement: at any time x on planet Earth, it is both day (on the light side of the planet) and also night (on the dark). One might also make arguments about the disjunctive's truth conditions during periods of twilight or dawning. 

* Strictly speaking a true disjunction is satisfied, i.e. evaluates as true, at least one of the conditions is true. Thus, the disjunctive is true if it is either day or night, or both day and night; and thus the disjunctive is true and the conjunctive is true. There is another logical operator called 'exclusive OR' that is like the disjunctive, but only satisfied if -- in the present case -- D is true and N is false, or alternatively D is false and N is true. It would not be satisfied if D and N are both true at the same time, as is really the case on Earth.