我所说的“生活必需品”是指人们通过自身努力所获得的，打一开始便是，或是经历长久使用，已经在生活中不可或缺的东西，除了野蛮人、穷困者和哲人，极少有人试图舍弃它们。对许多生灵来说，只有一种东西符合这一说法，那就是食物。草原上的野牛需要的不过几寸可口嫩草和能喝的水；除非它又寻求森林与大山的荫庇。除了食物与庇护之地，野兽别无所需了。而对于人类来说，当今气候下生活的必需品或许可以被精确地划分到食物、庇护所、衣物以及燃料这四个类目之下；因为只有确保了这些，我们才能自由且积极地开始满足自我的真实问题。人类所发明的，不只是房屋，还有衣物和烹饪；或许还意外地发现了火焰的温暖，于是将之为己所用。火焰打一开始还是个奢侈品，现如今围炉取暖已变得不可或缺。可以看到家猫家狗也同样习得了这样的第二天性。合适的房屋庇护和衣物遮盖使得我们得以保存身体内在的热量；可是这种庇护和遮盖一旦过量，也或是说“燃料”一旦过多，外在的热量就超过了内在，岂不成了一种“烹煮”？自然学家达尔文说他在火地群岛时，与同伴衣衫齐整地坐在火堆前尚且未脱寒意，却惊讶地发现那些原住民虽然赤身裸体远离火堆，但是“在如此炙烤下汗如雨下”。因此有人便说，澳洲土著赤身有恃无恐，欧洲人却裹着衣物打冷战。野蛮人的坚韧与文明人的智慧是否如水火一般不能相容呢？根据李比希的说法，人的身体是一个熔炉，食物是维持双肺之中燃烧的燃料。天冷时我们吃得多，天热时吃得就少些。动物的热量是缓慢燃烧的结果，而疾病意味着燃烧过快或燃料不足，要是通气不畅火焰熄灭，死亡便随之而来。当然这只是一种比喻，维生的热量并不能与火焰混为一谈。由此观之，“动物的生命”这样的说法，几乎与“动物的热量”同义。食物可以看作维持我们内部火焰燃烧的燃料，——而外在的燃料仅仅为了烹饪食物或是给我们的身体提供额外的温暖，庇护所与衣物也不过是为了维持由此生成和吸收的热量罢了。
By the words, necessary of life, I mean whatever, of all that man obtains by his own exertions, has been from the first, or from long use has become, so important to human life that few, if any, whether from savageness, or poverty, or philosophy, ever attempt to do without it. To many creatures there is in this sense but one necessary of life, Food. To the bison of the prairie it is a few inches of palatable grass, with water to drink; unless he seeks the Shelter of the forest or the mountain’s shadow. None of the brute creation requires more than Food and Shelter. The necessaries of life for man in this climate may, accurately enough, be distributed under the several heads of Food, Shelter, Clothing, and Fuel; for not till we have secured these are we prepared to entertain the true problems of Me with freedom and a prospect of success. Man has invented, not only houses, but clothes and cooked food; and possibly from the accidental discovery of the warmth of fire, and the consequent use of it, at first a luxury, arose the present necessity to sit by it. We observe cats and dogs acquiring the same second nature. By proper Shelter and Clothing we legitimately retain our own internal heat; but with an excess of these, or of Fuel, that is, with an external heat greater than our own internal, may not cookery properly be said to begin? Darwin, the naturalist, says of the inhabitants of Tierra del Fuego, that while his own party, who were well clothed and sitting close to a fire, were far from too warm, these naked savages, who were farther off, were observed, to his great surprise, “to be streaming with perspiration at undergoing such a roasting.” So, we are told, the New Hollander goes naked with impunity, while the European shivers in his clothes. Is it impossible to combine the hardiness of these savages with the intellectualness of the civilized man? According to Liebig, man’s body is a stove, and food the fuel which keeps up the internal combustion in the lungs. In cold weather we eat more, in warm less. The animal heat is the result of a slow combustion, and disease and death take place when this is too rapid; or for want of fuel, or from some defect in the draught, the fire goes out. Of course the vital heat is not to be confounded with fire; but so much for analogy. It appears, therefore, from the above list, that the expression, animal life, is nearly synonymous with the expression, animal heat; for while Food may be regarded as the Fuel which keeps up the fire within us,—and Fuel serves only to prepare that Food or to increase the warmth of our bodies by addition from without,—Shelter and Clothing also serve only to retain the heat thus generated and absorbed.
