All 3 Uses

positive
in
The Hitchhiker's Guide to the Galaxy
(Edited)

How can anyone maintain a positive mental attitude if you're saying things like that?
Chpt 7

positive = optimistic
"My God," complained Arthur, "you're talking about a positive mental attitude and you haven't even had your planet demolished today."
Chpt 7 *
Eventually, of course, after their Galaxy had been decimated over a few thousand years, it was realized that the whole thing had been a ghastly mistake, and so the two opposing battle fleets settled their few remaining differences in order to launch a joint attack on our own Galaxy—now positively identified as the source of the offending remark.
Chpt 31 *

positively = with certainty

Definitions:

(1) (positive as in: I'm absolutely positive!)
certain (having no doubt; or used for emphasis)
(2) (positive as in: had a positive effect)
good or beneficial
(3) (positive as in: positive feedback from my boss)
approval or agreement
(4) (positive as in: a positive attitude)
optimistic (expecting or focusing on good things); or agreeable
(5) (positive as in: The test came back positive.)
found a condition or substance to be present
indicating the presence of something being tested for -- especially a disease, condition, or substance
(6) (positive as in: a positive number)
greater than zero (of a number)
(7) (meaning too rare to warrant focus)
See a comprehensive dictionary for less common meanings of positive including some in the fields of electricity, physics, chemistry, medicine, philosophy, and grammar. Note that most all senses of positive indicate that something is good or present.