There is no remedy for love but to love more.
Source: The Journal of Henry David Thoreau, entry of 25 July 1839
Genuine Thoreau, from an early Journal entry of 1839 (the date is conventionally given as 25 July; I am confident of the Journal and the year, slightly less of the exact day). Quoted verbatim here, with no drift in the wording.
You must live in the present, launch yourself on every wave, find your eternity in each moment. Fools stand on their island of opportunities and look toward another land. There is no other land; there is no other life but this.
Source: The Journal of Henry David Thoreau, entry of April 1841
Genuine Thoreau, from a Journal entry of spring 1841 (usually dated 5 April 1841). The passage is quoted accurately, including the 'island of opportunities' image, which is the distinctive part and the reason I can place it with confidence. I am sure of the Journal and the year; the exact day is the conventional citation rather than something I can independently confirm.
There is no remedy for love but to love more.
Source: Journal of Henry David Thoreau, entry of 25 July 1839 (first published in The Writings of Henry David Thoreau: Journal, 1906)
Genuine Thoreau. The line appears in his Journal in an 1839 entry on love and friendship, and is carried in the standard Journal edition and on Thoreau's Wikiquote page as a sourced quotation. I am recalling the July 1839 date from memory and have not re-checked it against the printed Journal, so the date carries slightly more uncertainty than the attribution itself.
Nature is full of genius, full of the divinity; so that not a snowflake escapes its fashioning hand.
Source: Thoreau's Journal, February 1, 1852
Our life is frittered away by detail… simplify, simplify.
Source: Walden; or, Life in the Woods, Henry David Thoreau, 1854, ch. 2 ("Where I Lived, and What I Lived For")
Both fragments are genuine and come from the same famous passage in Walden ch. 2, but the ellipsis compresses several sentences. Thoreau's text runs: 'Our life is frittered away by detail... Simplicity, simplicity, simplicity! I say, let your affairs be as two or three... Simplify, simplify.' The condensation is faithful in sense but is not a contiguous quotation; if the corpus wants an exact citation, either restore the full passage or quote 'Simplify, simplify' on its own.
All change is a miracle to contemplate; but it is a miracle which is taking place every second.
Source: Walden, Henry David Thoreau, 1854 (chapter 'Higher Laws')
Genuine Thoreau, from the 'Higher Laws' chapter of Walden. The original reads 'a miracle which is taking place every instant' — the corpus text substitutes 'every second,' a common circulating variant. Recommend restoring 'instant' if the corpus prefers original wording.
Money is not required to buy one necessity of the soul.
Source: Walden, Henry David Thoreau, 1854 (chapter 'Conclusion')
Verbatim Thoreau from the 'Conclusion' of Walden. Short, exact, and correctly attributed.
If you advance confidently in the direction of your dreams, and endeavor to live the life you have imagined, you will meet with a success unexpected in common hours. You will pass an invisible boundary: new, universal and more liberal laws will begin to establish themselves around and within you and you will live with a higher order of beings. If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundation under them.
Source: Walden, Henry David Thoreau, 1854 (chapter 'Conclusion')
Both halves are genuine Walden, from 'Conclusion,' but the corpus text is the popular second-person paraphrase. Thoreau wrote in the third person: 'I learned this, at least, by my experiment: that if one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours.' The passage also compresses the original ('he will put some things behind, will pass an invisible boundary'; 'he will live with the license of a higher order of beings') and the castles line ends 'Now put the foundations under them,' plural. Source certain; wording modernized.
In the long run, we only hit what we aim at.
Source: Walden; or, Life in the Woods, Henry David Thoreau, 1854 (chapter 1, 'Economy')
Authentic Thoreau, from the 'Economy' chapter of Walden. The canonical wording is 'In the long run men hit only what they aim at. Therefore, though they should fail immediately, they had better aim at something high.' The corpus text has been silently modernized to 'we only hit what we aim at' — it preserves the meaning but is not what Thoreau wrote, and the second sentence, which carries the point, has been dropped. Recommend restoring the original wording before publication; the verdict is high-confidence on the source, with the caveat that the quoted text is a paraphrase of it.