In his attempt to furnish such an explanation, Engels implies that it was the complex thought engendered by manual labour and verbal language that spurred the development of our brains. In a biological sense, humans are not fundamentally different from other mammals, in that most mammals are only physically – and not cognitively – incapable of speech: “The dog and the horse, by association with man, have developed such a good ear for articulate speech that they easily learn to understand any language within their range of concept. Moreover they have acquired the capacity for feelings such as affection for man, gratitude, etc., which were previously foreign to them.