A logical necessity is a logical deduction which can be drawn from no premises, and is always true.

The most basic logically necessary argument follows this form:

Either it is raining, or it is not raining.

Or, in a more formal notation:

P v ~P

Which reads "P or not P". There are many other logically necessary deductions, but this is the most basic. Note that this is only true for some forms of logic; in fuzzy logic, for example, P & ~P may be a valid statement, if apparently non-sensical.