Is it better to use a positive or a negative command for a button, checkbox, etc.?

I have some checkboxes that should include/exclude some content on some reports.

But I am not sure if I should use:

Include all items

or

Exclude unused items

for the text of the checkbox.

What is better/makes more sense, etc.? Positive or negative button explanations?

Answer

Include all items, and here is why: People can compute positive words easier than negative ones.

For example: Do you want a part of my sandwich? or Do you not want most of my sandwich?