The two sources

"Suppose we have a man who refuses to consider the idea that a psycho-spiritual principle underlies all that is physical in the human being, who fails to understand, for instance, that the enlargement of a physical liver is the expression of something spiritual. Another man — stimulated by spiritual science, if you like — readily accepts the possibility that by penetrating into the realm of spirit one may arrive first at an inkling, then at faith, and finally at cognition and vision of spirit. Thus we have two men, one of whom rejects spirit, being satisfied with sense observation, while the other follows what we may call the will to achieve cognition of spirit. The one who refuses spiritual enlightenment will grow ever weaker, for he will be letting his spirit starve, wilt, and perish for lack of adequate nourishment which such enlightenment alone can provide. His spirit will lose strength — it cannot gain it; and everything that functions apart from this spirit will gain the upper hand and overpower him. He will become feeble in meeting all that takes place without his agency in his physical and etheric bodies. But the other, he who has the will to cognition, furnishes nourishment for his spirit which consequently gains strength and mastery over all that occurs independently in his etheric and physical bodies. — That is the most important point, and one which we shall presently be able to apply to a prominent case of our own day.
We know that upon entering the world the human being springs from two sources. His physical body is inherited from his ancestors, from his father and mother and their forbears. He inherits certain traits, good or bad, that are simply inherent in the blood, in the line of descent. But in every case of this sort the forces a child brings along from his previous incarnation unite with these inherited qualities. Now, you know that today a great deal is talked about “hereditary tendencies” whenever some disease or other makes its appearance. How this term is abused nowadays — though it is quite justified within a narrow scope! Whenever anything crops up that can be proved to have been an attribute of some ancestor, hereditary tendencies are invoked; and because people know nothing of active spiritual forces derived from the previous incarnation they endow these inherited tendencies with overwhelming power. If they knew that a spiritual factor accompanied us from our previous incarnation they would say, Well and good: we believe absolutely in hereditary tendencies, but we know as well what stems from the previous incarnation in the way of inner, central soul forces, and that if sufficiently strengthened and invigorated these will gain the upper hand over matter — that is, over hereditary tendencies. — And such a man, capable of rising to the cognition of spirit, would continue: No matter how powerfully the inherited tendencies affect me, I shall provide nourishment for the spirit in me; for in this way I shall master them. — But anyone who does not work upon his spiritual nature, upon that which is not inherited, will positively fall a prey to inherited tendencies as a result of such lack of faith; and in this way materialistic superstition will actually bring about a steady increase in their power over us. We shall be engulfed in the quagmire of hereditary tendencies unless we fortify our spirit and, by means of a strong spirit, vanquish each time anew whatever is inherited.
In our time, when the consequences of materialism are so formidable, you must naturally still guard against overestimating the power of spirit. It would be a mistake to object, If that were the case, all anthroposophists would be bursting with health, for they believe in the spirit. Man's position on the earth is not only that of an individual being: he is a part of the whole world; and spirit, like all else, must grow in strength. But once spirit has become debilitated, as at present, it will not at once affect even the most anthroposophical of men — no matter how much nourishment he furnishes the spirit — to such an extent that he can overcome what springs from material sources; yet all the more surely will this tell in his next incarnation, as expressed in his health and strength. Men will grow weaker and weaker unless they believe in the spirit, for otherwise they deliver themselves over to their inherited tendencies. They themselves have effected this weakening of their spirit, because everything here concerned depends upon their attitude toward spirit.
Nor should one imagine it an easy matter to correlate all the conditions here involved..."

Rudolf Steiner, 1909
GA 112