it is impossible of attainment = it is impossible to attain

通过分析以下COCA类似例句，我们可以得知形容词impossible的一个用法，即：

Sth is impossible of + 表示动作意义的抽象名词 = Sth is impossible to do (悬空不定式）。例如：

Actions of animals are impossible of explanation as due to instinct.

= Actions of animals are impossible to explain as due to instinct.

Any ethical goal that is conceptually impossible of achievement is an absurd goal.

= Any ethical goal that is conceptually impossible to achieve is an absurd goal.

在以上二个句义相等的结构中，悬空不定式或者of介词短语都是表语形容词impossible的不足语。

以下为COCA部分类似例句：

At first Fox was pessimistic, considering the Serra' as impossible of conquest by a locomotive as the Himalayas'.

Or the problem may be so specialized and varying in nature as to be impossible of capture within the boundaries of a general rule.

This intellectualizing is perhaps akin to this author's going far afield in his verbose analysis, something like schizophrenic " word-salad ": Nearly impossible of 

comprehension.

 After the instances I have cited of actions of animals which are impossible of explanation as due to instinct, Mr. Burroughs may reply: " Your instances are easily explained by the simple law of association. "

...it is something impossible of describe.

Surely any ethical goal that is conceptually impossible of achievement is an absurd goal.