Animal nature
Locke uses regular old animals as an analog for the human experience. This is strikingly effective, because scientifically speaking, humans literally are animals. But, during this season of philosophy, such ideas weren't necessarily taken for granted. Through the insight that human consciousness shares intentions with animals, the human experience is oriented in a proper framework. He points out that hunger and sexual desire are shared between animals and humans.
Food and perception
Food is offered as a symbol for human perception. Instead of merely seeing an apple and thinking, "That is an object," a human being sees an apple and thinks, "I want to eat that." This sense perception is clearly rooted in animal instinct, showing that much of human consciousness is merely the mind observing the natural unfolding of animal instincts. The perception of reality is shown as more mysterious than originally understood; it cannot be taken for granted.
Language games
One obvious difference between humans and animals is found in language. Through the matrix of words that are shared among participants of the same language, there are language games that unfold. Some of these are philosophical, and others are religious or theoretical. Economic intentions are often construed in language, and so on. The point here is that language can be seen as a collective symbol for all that separates humans from animals. Humans are like animals, except that language allows for a dimension of theory and abstract conception, says Locke.
Human essence
The question of the book can be seen as an attempt to understand human essence. This is simply a reflection of the question, "What separates humans from animals?" The essential difference, if isolated, could be considered further for its implications and presuppositions. This is a common philosophical aim of Locke's time, to isolate the human essence. He arrives at an answer very close to linguistic philosophy.
Science as ontology
Locke addresses the school of Ontology, which is the philosophical pursuit of describing existence and categorizing the way reality exists. Locke suggests science as a first-step in that direction. Without addressing whatever could be known through scientific inquiry, ontology is nothing more than language applied to hypothetical situations. By choosing science first and philosophy second, Locke hopes to orient Ontology in literal fact, so that it isn't merely speculative.
