Emotions change relatively slowly over time (compared to sensations, which are instantaneous). For example, fear and anger are lasting emotions, so are joy and sorrow. The changes in emotions are generally triggered by the animat's sensations. An emotion could also correspond directly to a single sensation (for instance, surprise).
Each primary emotion may have variable intensity (for instance, fear, as shown in Table 37.3). We term each of the different values secondary emotions (for instance, terror and apprehension). Primary emotions may also be defined such that two of them may not be present at the same time; we call these complementary emotions.
A mood will generally be represented as a set of primary emotions, each with many possible states: the secondary emotions. For example, a good mood might be a blend of acceptance (primary), satisfaction (secondary to joy), and awe (secondary to surprise). Such combinations of primary emotions produce complex emotions (for instance, love and optimism).
Feelings are a more powerful concept; unlike emotions, they can be expressed about the past or future, and unlike sensations they do not rely directly on current state. Instead, feelings can be associated with arbitrary objects, in the past or future.
Feelings are also very broad in that they can be applied to any concept (for instance, attributes or categories of objects). Examples of feelings are disgust with varieties of food, hate for different types of car, or a phobia of moving obstacles (see Table 37.4).