Both Uses of

positive
in
A Thousand Acres

And then there was the willful positive thinking, the self-induced illusion that everything would turn out fine, when we had all kinds of evidence that it wouldn't.†
p. 154.8 *

positive = optimistic

Unless he positively commits himself—" "But he has!†
p. 300.9 *

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)
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.