Rational Agent: [Good behaviour (The Concept of Rationality)]:

A rational agent is one that does the right thing. by considering the consequences of the agent's behaviour. Design performance measures according to what one wants in the environment.
What is rational at any given time depends on four things:

Definition: For each possible percept sequence, a rational agent should select an action that is expected to maximize its performance measure, given the evidence provided by the percept sequence and whatever built-in knowledge the agent has.

Task Environment: To design a relational agent, we must specify the performance measure, the environment, and the agent’s actuators. We group all these under the heading of the task environment. For the acronymically minded, we call this the PEAS, where P stands for Performance, E stands for Environment, A stands for Actuators and S stands for Sensors. In designing an agent, the first step must always be to specify the task environment as fully as possible.