I think Edgar Allan Poe’s Eureka is one of the most underrated works of speculative philosophy:
Books in Brief — Edgar Allan Poe: Eureka | by Adesh Acharya | Medium
Published in 1848, this short work is where Poe searches for the sweetest-spot of wisdom to look into scientific and philosophical questions.
In his own words, he wanted it to be considered as a work of poetry:
To the few who love me and whom I love, to those who feel rather than think, to the dreamers and those who put their faith in dreams as in the only realities, I offer this book of truths, not in its character of truth-teller, but for the beauty that abounds in its truth, constituting it true. (Preface)
Apart from physical, cosmological, and spiritual implications, Poe in this work provides us with his epistemology. By that I mean — he provides us the method/way he believes in and has used to observe what he has observed and conclude what he has concluded. In doing so he has also provided us with his own brief interpretation of the history of philosophy.

I have summarized in this manner:
It was the metaphysicians who first came up with singular fancy that there exist but two practicable roads to Truth. Aristotle was the founder and popularizer of the deductive or the apriori method. He started with axioms, or self-evident truths and from axioms he proceeded logically, to results. His most illustrious disciples were one Euclid and Kant. Aristotle and his method reigned supreme until James Hogg preached an entirely different system, which he called the à posteriori or inductive method.
His plan referred altogether to sensation. He proceeded by observing, analyzing, and classifying facts — instantiæ Naturæ, as they were somewhat affectedly called — and arranging them into general laws.
While the mode of Aristotle rested on noumena, that of Hogg depend on phenomena; and so great was the admiration excited by this latter system that, at its first introduction, Aristotle fell into general disrepute.
But he recovered ground, and was permitted to divide the empire of Philosophy:
But these method retarded the progress of true Science, which makes its most important advances by seemingly intuitive leaps.
This way, investigation was similar to crawling and for many centuries,
For many years, it didn’t matter whether the truth was even demonstrably such, for the dogmatizing philosophers of that epoch regarded only the road by which it professed to have been attained. It all ended with the scrutiny of the means, where it was found that the mean fit neither under Hog, nor under Aristotle.
If the crawling system was exclusively adopted, men wouldn’t have arrived at the maximum amount of truth because the repression of imagination was an evil not to be counterbalanced even by absolute certainty in the snail processes. Nor was that certainty absolute. Their method was like holding something close to the eyes to see it better. Which in turn blinded the seers.
The major taint in Baconianism lay in its tendency to throw power and consideration into the hands of merely perceptive men who mostly dug for minute facts, especially in physical science. All they did was depended on facts and closed their eyes to everything else. They gave hard time to those who wanted to evolve from facts through generalization. They called them ‘theoretical,’ ‘theory,’ ‘theorist’ in a degrading manner.
On the other hand, the Aristotleians were blind as they had:
The focus was a lot on Logic. A certain Mill said that the ability or inability to conceive is in no case to be received as a criterion of axiomatic truth.
But their logic was baseless, worthless and fantastic altogether. The two narrow and crooked paths then — the one of creeping and the other of crawling —is where they confined the Soul:
This way, none of them came — even by accident — to the broadest, the straightest and most available of all mere roads — the majestic highway of the Consistent. They failed to deduce from the works of God the vitally momentous consideration that a perfect consistency can be nothing but an absolute truth?
After that proposition, the process of truth investigation was taken out of the hands of the ground-moles and given to the only true thinkers — to the generally-educated men of ardent imagination:
The speculators and the theorizers. The Keplers, The Laplaces, whose theories are corrected/reduced/sifted/cleared of their chaff of inconsistency —
This new method is powerful and it is proved by the fact that Newton’ s gravitation was deduced from Kepler and Kepler being a speculator/theorizer had merely guessed it.
A conviction resulting from shadowy deductions or inductions.
Elsewhere, he describes an artist as someone with an exquisite sense of beauty which affords him not only a rapturous enjoyment but also a sense of deformity of disproportion (FIFTY SUGGESTIONS XXII).
Poets (who are artists) have the ability to sense the wrong and they can see injustice where the unpoetical see none. They have a clear-sightedness in respect to wrong which is nothing more than a corollary from the vivid sensation of right. Poets have an irritability towards the wrong.