Thought is a reflection of essential attributes and internal laws of objective reality in conscious, indirect and generalization by human brain with consciousness [6]. In recent years, there has been a noteworthy shift of interest in cognitive science. Cognitive process rises man's sense perceptions and impressions to logical knowledge. According to abstraction degree of cognitive process, human thought can be divided into three levels: perception thought, image thought and abstraction thought. A hierarchical model of thought which illustrates the characteristics and correlations of thought levels has been proposed in [7].

Perception thought is the lowest level of thought. Behaviour is the objective of research in perception thought. Reflection is a function of stimulus. Perception thought emphasizes stimulus?reflection schema or perception action schema. The thought of animal and infant usually belong to perception thought because they can not introspect, and also can not declare empirical consciousness [6]. In perception thought, intelligent behaviour takes place without representation and reasoning.

Behaviour-based artificial intelligence has produced the models of intelligence which study intelligence from the bottom up, concentrating on physical systems, situated in the world, autonomously carrying out tasks of various sorts. They claim that the simple things to do with perception and mobility in a dynamic environment took evolution much longer to perfect. Intelligence in humen have been taking place for only a very small fraction of our evolutionary lineage. Machine intelligence can take evolution by the dynamics of interaction with the world. Image thought adopts intuitive imagery as thinking element. Intuitive imagery is one kind of information which acquires through processing perceptual representation, but does not yet generate concepts of language. Typical image thought is pattern recognition which can deal with pattern information, such as character, image, speech, classification and recognition of objects, and so on [6].

Based on perceptual knowledge, the process which reflects the common properties and exposes internal relations of distinct objects through concepts, judgement and inference is called abstraction thought [6]. Concepts are no longer the phenomena, the separate aspects and the external relations, while reflect the essences and internal relations of objects. Judgement represents the certain relations between conceptions. Inference acquires new knowledge from existing knowledge. There are existing deductive reasoning, inductive reasoning, and abductive reasoning currently. By means of judgement and inference one is able to draw logical conclusions. Logical knowledge is capable of grasping the development of the surrounding world in its totality, the internal relations of all its aspects.

Attention focuses consciousness to produce greater vividness and limits the number of thoughts that can be entertained at one time. Attention forces human thinking process from parallel to sequential in terms of leaping from image thought to abstraction thought.