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.
偶然性取决于机会的情形；不确定性 The condition of being dependent on chance; uncertainty.
