Positive and Good

How did 'positive' come to mean 'good' and 'negative', 'bad'?

Few people would want a positive result after being screened for cancer--right?

I suppose the primary meaning I have in mind for the word is something like 'active' or 'existing'. For instance, a positive dislike for peanut butter means that someone doesn't merely lack a taste for peanut butter, but actively dislikes it. We might say that 'positive' is convex whereas 'negative' is concave; 'positive' is presence and 'negative' is absence. In fact, entering "positive dictionary" into a well-known search engine that has perhaps turned the world into a panopticon, I find the first result is:

"... consisting in or characterized by the presence rather than the absence of distinguishing features."

The online Cambridge Dictionary has first a version of what I think is the most common modern usage of the word:

 "... full of hope and confidence, or giving cause for hope and confidence".

The Etymology Online Dictionary emphasises, at least in terms of priority, a different meaning:

"... early 14c., originally a legal term meaning 'formally laid down, decreed or legislated by authority' (opposed to natural),  from Old French positif (13c.) and directly from Latin positivus 'settled by agreement, positive' (opposed to naturalis 'natural'), from positus, past participle of ponere 'put, place' (see position (n.))."

I have begun to muse on its meaning recently and this musing was brought into a sharper curiosity by this quote on the front of a book I've started reading:

"This wonderfully positive and vivid history is a delight on every page."

I wonder what sense of 'positive' is intended here. The book is a history of poetry. I take there to be two possible meanings, either or both of which might be intended, one of which might be called 'specific' and one 'general'. The specific meaning would be that the book displays a favourable or encouraging attitude towards the value of poetry. The general meaning would be that the book is cheerful or uplifting in tone. 

This is not quite, of course, the 'positive = good' meaning that I have mentioned at the beginning of this blog post, but I am reminded of it. For instance, though we can't simply replace the word 'positive' with the word 'good' in the sentence, we could paraphrase it as something like: "This history, which is wonderfully vivid and clearly conveys a sense that poetry is good, is a delight on every page." 

Or perhaps this paraphrase is misleading. Is there is clear sense in which the word positive is being used here? The word 'positive', when axiological, seems to slide between meanings almost as if the person using does not quite know himself what his (Bernard O'Donoghue, in this case) intended meaning is.

I think a clearer example of where 'positive' tends to replace the word 'good' is the following:

"I'm glad to hear the news is good." "I'm glad to hear the news is positive."

I wonder by what route or process this meaning has come about.

I doubt that it has arisen directly from "formally laid down, decreed or legislated by authority", although, now that I've written that, I'm not so sure. There is a sense in which 'positive' now seems to connote the authority of popular consensus (laid down, but not quite formally). 

What I suspect is that the word, when used to mean 'good' and other variations of this pole of the axiological spectrum, has a secret link with my own primary association with the word: positive presence, which is to say, existence. If this is true, when people are using the word in this way, they are tacitly endorsing the privation theory of evil. I have some sympathy with this theory, but I wonder whether those who generally use the word 'positive' in this way would continue with this endorsement if they thought it through.

Incidentally, the book with the quote on its front is A Little History of Poetry by John Carey. I'm 14 pages in. It looks like being a very informative and fascinating read.