The text defines psychology as the scientific investigation of behaviour and:

The text defines psychology as the scientific investigation of behavior and mental processes.

Here's a step-by-step breakdown:

1. "Scientific investigation" refers to the systematic and objective approach used by psychologists to understand and interpret how we think, feel, and behave.

2. "Behavior" refers to any observable action made by a living person or animal. This can include physical actions, as well as things like speech or facial expressions.

3. "Mental processes" refers to the internal, subjective experiences we have, such as thoughts, feelings, and perceptions.

So, in essence, psychology is the study of how and why we behave the way we do, both externally (through our actions) and internally (through our thoughts and feelings).