I think he means that "life" cant be defined. But life is just a word, it can be defined if its willed so.
But we clearly dont have a commonly accepted definition.
Yes; it is a moral form, like time, or justice, or even existence, and attempt as we may we can never de-fin-e what is in-fin-ite... If we say justice instead of life, what ever the difference between these two is, then no matter how many instances we have of justice we have not the thing JUSTICE; which is not thing at all, but a moral idea only... We can define life in terms of justice, and justice in terms of liberty, and liberty in terms of virtue generally; and yet be no closer to a definition of any of these moral forms... We have examples of life as examples of liberty, and but for those examples they would be only words... But more than examples is needed to define a form... It takes all the examples to define a moral form... Of physical forms the case is easier... The form "Dog" includes all dogs, and yet all the dogs is the definition of the form, and if a dog were to deviate enough from the form it would create a whole new form, and short of that would only create a catagory, such as three legged dogs... Do you get my point... Moral forms have no specific reality, but are more spiritual qualities having physical significance but no true meaning... "Dog" has a certain fixed meaning as life or existence cannot...