With regard to mental deficiency, we think it possible to formulate precise definitions which will enable all competent persons to agree as to the diagnosis of idiocy, imbecility, and feeble-mindedness. We are aware that in making this statement we are running counter to the general practice of medical alienists. When these, in an admission certificate, call a child "idiot," "feeble-minded," or "imbecile," they are rarely in agreement with the confrère who, a few days later, examines the same child, and makes a new diagnosis. We have made a methodical comparison between the admission certificates filled up for the same children with a few days' interval by the doctors of Sainte-Anne, Bicêtre, the Salpêtrière, and Vaucluse. We have compared several hundreds of these certificates, and we think we may say without exaggeration that they looked as if they had been drawn by chance out of a sack. This is a fact which many alienists have already suspected, and Dr. Blin[11] has expressed himself frankly on the subject. What is the cause of such contradictions? They result, in great measure from the use of ill-defined terms. To the majority of alienists, the idiot is one who is _profoundly_ affected in his mental faculties, the imbecile is _a little less_, and the feeble-minded _less still_. What mean those words: "profoundly," "a little less," "less still"? No one defines them. They are taken to be indefinable. It is no wonder they are understood so differently. All this trouble would disappear if the following definitions were adopted: DEFINITION OF AN IDIOT. _An idiot is any child who never learns to communicate with his kind by speech--that is to say, one who can neither express his thoughts verbally nor understand the verbally expressed thoughts of others, this inability being due solely to defective intelligence, and not to any disturbance of hearing, nor to any affection of the organs of phonation._ Since a normal child of two years of age can understand the speech of others, and can make itself understood by others, so far as its simple wants are concerned, it is evident that the distinction between an idiot and a normal child is easily made. DEFINITION OF AN IMBECILE. _An imbecile is any child who fails to learn how to communicate with his kind by means of writing--that is to say, one who can neither express his thoughts in writing, nor read writing or print, or, more correctly, understand what he reads, this failure being due to defective intelligence, and not to any defect of vision or any paralysis of the arm which would explain his inability._ One will not count a child an imbecile until he has had much more than the normal time to learn to read and write. The normal time in schools is six months. A child who does not yet know his letters after being at school for two years is likely to be an imbecile.