“It is natural for children to be fed with mother’s milk”. This seems to be biologically as well as historically right. As shown in such cases, naturalness often has normative connotation and becomes the basis of belief that something is true and good. However, a detailed analysis of the content, the reasons and the background in which the word “nature” is used reveals that it has varied dramatically over time. This article will explore its historical change in Japan in terms of mother’s milk or breastfeeding, from the early modern to the contemporary period, mainly by investigating the description of child-rearing books from each era. The historical variability of “nature” is paradoxically due to the fact that it is always supposed to be unchanging and serves as the “ground” of truth and goodness. In every age, our lives are beset by various difficulties and problems, and we repeatedly return to “nature” to get out of our predicament. However, what is “natural” cannot be determined from the beginning. It is only in the social and historical context that its meaning finally becomes concrete in the light of the problems we face. Whatever we consider “natural,” the culture of human society always has something contrary to it. This is especially true today, when our lives are so far removed from naturalness. In such an age, “nature” becomes a strong support for people, especially in areas such as child-rearing, where it is difficult to find reliable guidelines.