模态逻辑命题的限制条件，建立在其内容的肯定或否定其可能性，不可能性，偶然性或必然性基础之上 The classification of propositions on the basis of whether they assert or deny the possibility, impossibility, contingency, or necessity of their content.
必然发生或结局的确定性；必然性 Sureness of occurrence or result; inevitability.
