How much of what you experience did you actually choose to notice?
What it argues
Attention is the mind taking possession of one of several simultaneously possible objects. James's claim is that experience is not given but selected: my experience is what I agree to attend to. What you call your personality is partly a settled pattern of what you notice and what you let pass.
The probe
For one hour, every time your attention moves, write one line: what pulled it, something outside or a thought of your own.
The source
The Principles of Psychology, ch. XI — William James, 1890. The link goes to the text or to a catalogue record for it, not to a summary. This one is the text itself.