Labriola, moreover, has a special merit, which marks him off from the ordinary exponents and adapters of historical materialism. Although his theoretical formulae may here and there expose him to criticism, when he turns to history, _i.e._ to concrete facts, he changes his attitude, throws off as it were, the burden of theory and becomes cautious and circumspect: _he possesses, in a high degree, respect for history_. He shows unceasingly his dislike for formulae of every kind, when concerned to establish and scrutinise definite processes, nor does he forget to give the warning that there exists 'no theory, however good and excellent in itself, which will help us to a summary knowledge of every historical detail.'[49] In his last book we may note especially a full inquiry into what could possibly be the nature of a _history of Christianity_. Labriola criticises those who set up as an historical subject the _essence_ of Christianity, of which it is unknown where or when it has existed; since the history of the last centuries of the Roman Empire shows us merely the origin and growth of what constituted the Christian society, or the church, a varying group of facts amidst varied historical conditions. This critical opinion held by Labriola seems to me perfectly correct; since it is not meant to deny, (what I myself, do not deny) the justification of that method of historical exposition, which for lack of another phrase, I once called _histories by concepts_,[50] thus distinguishing it from the historical exposition of the life of a given social group in a given place and during a given period of time. He who writes the _history of Christianity_, claims in truth, to accomplish a task somewhat similar to the tasks of the historians of _literature_, of _philosophy_, of _art_: _i.e._ to isolate a body of facts which enter into a fixed concept, and to arrange them in a chronological series, without however denying or ignoring the source which these facts have in the other facts of life, but keeping them apart for the convenience of more detailed consideration. The worst of it is that whereas literature, philosophy, art and so on are determined or determinable concepts, Christianity is almost solely a bond, which unites beliefs often intrinsically very diverse; and, in writing the history of Christianity, there is often a danger of writing in reality the history of a _name, void without substance_.[51]