Properties and methods are given "positive" names as a matter of convention, and to make reasoning about Nil easier.
In Objective-C, the question "should I name my method isEnabled or isDisabled?" has a reasonable answer - you consider what makes most sense (or any sense) when called on Nil.
This is probably a good rule for other languages too - "negative" booleans can lead to double-negatives and unreadable code.
