That is Henry's description of Walden Pond in the chapter of Walden called "The Ponds." Walden, as he said, is noteworthy for two things (among others): the purity of its water and the fact that it has no surface inlets. Indeed, these two features are surely connected. Any given water molecule in Walden has been sitting on the surface of the earth for a relatively short period of time. It has not long been wandering from lake to lake, exposed to light, air and nutrients. The result is a gem-clear lake. I have noticed this phenomenon many times in other lakes. The lake you see behind me in my blog portrait in the left sidebar is one I have visited many times since 1969. It is perched atop a mountain of granite, fed only by melted snow. To the the right is a view of the lake, looking down into it. The shape you see near the bottom of the picture is a large boulder at the bottom of the lake. (Click to enlarge.) The lake has algae, but no weeds, no macroscopic plants at all. In fact, my lake is much "purer" than Walden, as it doesn't even contain any fish.
The scenery of Walden is on a humble scale, and, though very beautiful, does not approach to grandeur, ... yet this pond is so remarkable for its depth and purity as to merit a particular description. It is a clear and deep green well, half a mile long and a mile and three quarters in circumference, and contains about sixty-one and a half acres; a perennial spring in the midst of pine and oak woods, without any visible inlet or outlet except by the clouds and evaporation.
Maybe you can now guess what my point is: Where bodies of surface water are concerned, purity is death.
What is purity, after all? The idea of purity rests on three more fundamental ideas. To have a concept of purity you must, first, have the idea of a type of material as properly belonging in a certain domain (eg., domain: lake; material: water). Second, you need the idea that everything else is (eg., anything that can obstruct my view of the bottom) foreign matter and does not belong here. Finally, when this foreign matter invades the material that belongs here, what you have (third fundamental idea) is pollution (the Greeks called it miasma).
Aesthetically, purity is a very attractive idea. Anyone who has looked into a flawless gem or a pure (ie., dead) body of water knows this. But I see deep ethical and political problems with the idea, possibly fatal ones. Purity is, first of all, a entirely negative. To achieve purity is simply to chase certain things out, or annihilate them. Second, it is profoundly reactionary. If purity is ever achieved, all changes in regard to it are by definition bad and to be resisted. So its ideal end state is stasis stillness. In view of that it is no surprise the purity and death go hand in hand. Finally, if the concept is in a way profoundly intolerant. The idea of purity involves splitting the world up into the element that belongs here and the foreign element that does not, and should be chased back home or wiped out of existence.
We associate the idea of purity with ascetics, and so we should. Ascetics are deeply negative people, obsessed with chasing out the bad thoughts, flushing out their bad feelings, avoiding "wrong" foods and, in extreme cases, flushing "impurities" out of their very bowels. But it also is of great interest to a certain sort of environmentalist. After all, as I have said, pollution is nothing but the introduction of impurities. At their most purity-obsessed extreme, environmentalists can be very negative, reactionary, and intolerant people. Finally, and less obviously, purity is an idea that is very congenial to racists. Indeed, you could almost define racism as nothing more or less than the application of the purity-ideal to race. I can't really weigh in on the question of whether the Third Reich was the first consciously environmentalist government, but I can say there is a connection between Nazi racism and Nazi conservationism: they share a common theme in the idea of purity. Note that the Nazis referred to racial intermarriage as "race-pollution." To a racist, that is exactly what it is.
Thoreau's hunger for purity is both attractive and worrisome. (On both points, see the opening pages of the Walden chapter, "Higher Laws".) The more I think about it, though, the more I want to resist my own tendency to seek purity. And to exhort others: Oh my brothers and my sisters, be positive, be tolerant, commit pollution! Dare to be impure!