5 ideas
Haldane observes that if you drop a mouse down a thousand-yard mine shaft, it gets a slight shock on landing and walks away, provided the ground is fairly soft. A rat is killed, a man is broken, and a horse splashes. Air resistance scales with surface area while weight scales with volume, so the smaller the animal, the more its fall is braked relative to its mass.
When an animal is scaled up, its weight grows with the cube of its length, but the strength of its bones grows only with the square. Haldane's example is a giant ten times human height, whose thigh bones would break under his own weight at every step. Large animals must therefore have disproportionately thick bones, and surface-dependent functions like breathing, absorption and heat loss need folded lungs, intestines and other elaborate structures. For every type of animal there is a most convenient size, and a large change in size inevitably carries a change of design.
Haldane extends the biological principle to human institutions: just as each animal has an optimal size, so does each form of government. Greek-style direct democracy suited a small city-state where every citizen could hear the speakers, but it does not scale unchanged to a large nation. Proposals for social organization should be judged by whether they can work at the scale actually intended, not only by their merits in principle.
Human conceptions of reality are built from the particular senses and bodily scale evolution gave us, as other animals' very different sensory worlds show. Haldane therefore suspects that the universe is not only queerer than we suppose, but queerer than we can suppose. Our failure to imagine an alternative world is weak evidence that it cannot exist, and science repeatedly uncovers realities that no prior imagination anticipated.
Haldane defends self-experimentation, as in his own work swallowing ammonium chloride and similar substances to alter his blood chemistry and study the effects. A human subject can report sensations and symptoms that no animal can describe.