The Greek phrase ei mē ti is an exceptive clause meaning 'unless' or 'except.' It is a strengthened form of ei mē (if not), with ti adding specificity — 'if not something.' It limits or qualifies a statement by introducing an exception.
Paul uses ei mē ti in 1 Corinthians 7:5 regarding the sexual relationship in marriage: 'Do not deprive one another, except (ei mē ti) perhaps by agreement for a limited time.' The exception clause protects both the principle (mutual availability) and the legitimate use of temporary abstinence for prayer. The grammar of exception teaches that biblical commands have carefully considered limits — wisdom knows not only the rule but the exception.