In this sentence a semicolon is quite indispensable in order to disconnect the second group beginning with “that” from “mean,” to which the preceding similarly formed group, beginning with “that,” belongs.
Between these extremes are many sentences which may take either the comma or the semicolon without distractive effect. The close punctuator will generally prefer the semicolon; the open punctuator, the comma. (For a discussion of close and open punctuation, see Chapter XV.)
The degrees of variation in the relationship between parts of language are so great that a differentiation between the comma and the semicolon is at times almost impossible. Fortunately, a quite indiscriminate use of these two marks in the class of sentences under consideration is not always misleading or distractive to the reader; but the indiscriminate use of marks tends to lessen the importance attached by a reader to punctuation.
EXAMPLES
1. He suffered much, and he also suffered long.
2. Virtue is intolerant of vice, and virtue is just as contagious as vice ever was.
3. Be the first to say what is self-evident, and you are immortal.
4. In some states the legislatures meet annually, and in others biennially.
4-1. In some states the legislatures meet annually; in others, biennially.