A really dominant instinct is a veritable source of psycho-physical energy, unifying and maintaining in vigour all the activities directed to its fulfilment.[74] A young man in love is stimulated not only to emotional ardour, but also to hard work in the interests of the future home.

The explorer develops amazing powers of endurance; the inventor in the ecstasy of creation draws on deep vital forces, and may carry on for long periods without sleep or food.

If we apply this law to the great examples of the spiritual life, we see in the vigour and totality of their self-giving to spiritual interests a mark of instinctive action; and in the power, the indifference to hardship which these selves develop, the result of unification, of an "all-or-none" response to the religious or philanthropic stimulus.

It helps us to understand the cheerful austerities of the true ascetic; the superhuman achievement of St.Paul, little hindered by the "thorn in the flesh"; the career of St.Joan of Arc; the way in which St.Teresa or St.Ignatius, tormented by ill-health, yet brought their great conceptions to birth; the powers of resistance displayed by George Fox and other Quaker saints.

It explains Mary Slessor living and working bare-foot and bare-headed under the tropical sun, disdaining the use of mosquito nets, eating native food, and taking with impunity daily risks fatal to the average European.[75] It shows us, too, why the great heroes of the spiritual life so seldom think out their positions, or husband their powers.