Impossibility of performance指的是“无法履行（合同等）；履行不能”，它区别于履行困难〔difficulty of performance〕，是指因事物本身不可能完成，而非指当事人没有履行能力。它分为自始履行不能〔original impossibility〕和意外履行不能〔supervening impossibility〕，前者指在合同缔结时即已存在的无法履行的情形，后者指在合同缔结后发生其他情形而造成的合同无法履行。impossibility of performance of contract指的就是“合同的无法履行；合同履行不能”，合同当事人在并非由于其过错而使合同履行成为不可能或完全不能实行〔totally impracticable〕的情况下，可据此免于履行义务的一项原则。作为免于履行的抗辩事由，是指由于下述情况而无法履行，例如：合同主体内容的破坏、履行所必须的人员的死亡、合同约定的履行行为成为非法等。

参考例句：Issues such as spent treaties, supervening impossibility of performance, dispute resolution, interpretation, termination or withdrawal were all dealt with to some extent by reference to the original treaty provisions or to the residual rules of the Vienna Convention on the Law of Treaties and parallel rules of customary international law.

诸如失效的条约、发生意外不可能履行、争端解决、解释、终止或撤销等问题的处理都在一定程度上参照《维也纳条约法公约》的原条约规定或备用规则以及习惯国际法的并行规则。

The present draft articles are without prejudice to the termination, withdrawal or suspension of treaties as a consequence of, interalia: (a) a material breach; (b) supervening impossibility of performance; or (c) a fundamental change of circumstances.

本条款草案不妨碍尤其因下列情况而终止、退出或中止条约：(a)发生重大违约情事；(b)发生意外不可能履行；或(c)情况的根本改变。

Thus some of the modern State practice refers, for the most part, to the effect of a fundamental change of circumstances, or to the supervening impossibility of performance, and is accordingly irrelevant.

例如，一些现代国家的做法，多半是指情况发生根本变化的影响或履行不可能，因此无关。