[25] Braulio’s list mentions a _Liber de Haeresibus_ which does not appear in Arevalo’s edition, and fails to mention the _Liber de Ordine Creaturarum_ and the _Epistolae_, which are included. Ildephonsus’s list is still less complete, leaving out the _Proœmia_, _Allegoriae_, _Numeri_, _Officia_, _Regula_, _De Ordine Creaturarum_, _Chronicon_, _De Viris Illustribus_, and the _Epistolae_. For the present purpose, which is to ascertain something of the intellectual outlook of the dark ages, the _Etymologiae_ is, of course, of prime importance, since it contains in condensed form nearly everything that Isidore has written elsewhere. A passing attention, however, should be given to some of his other works, especially those of the more secular sort, in which his characteristic ideas are frequently developed with greater fullness than in the _Etymologies_ itself. These include in particular the _Differentiae_, the _De Natura Rerum_, the _Liber Numerorum_, the _Allegoriae_, the _Sententiae_, and the _De Ordine Creaturarum_. The _Differentiae_ is in two books, the first of which treats of differences of words, and the second, of differences of things. The plan of the first book is alphabetical; words are ranged in pairs and distinguished from each other. Usually these words are synonyms, and directions are given for their proper use; as, _populus_ and _plebs_, _recens_ and _novus_, _religio_ and _fides_; but frequently words of similar sound are distinguished; as, _vis_ and _bis_, _hora_ and _ora_, _hos_ and _os_, _marem_ and _mare_. From these latter valuable hints on the Latin pronunciation of the time may be obtained. The second book, _On Differences of Things_, treats in a brief way of such distinctions as those between _deus_ and _dominus_; between the nativity of Christ and of man; between angels, demons, and men; angelic and human wickedness; _animus_ and _anima_; the grace of God and the will of man; the life of action and that of contemplation. The introductory remarks of the _Differentiae_ are worth translating, since they reveal one of the most marked characteristics of Isidore’s thinking, the stress that he laid on words. They are as follows: Many of the ancients sought to define the differences of words, making some subtle distinction between word and word. But the heathen poets disregarded the proper meanings of words under the compulsion of metre. And so, beginning with them, it became the custom for writers to use words without proper discrimination. But although words seem alike, still they are distinguished from one another by having each an origin of its own.[26] Cato was the first of the Latins to write on this subject,[27] after whose example I have in part written myself of a very few, and have in part taken them from the books of the writers.[28] [26] Quadam propria origine.