There are many different definitions of children's literature and even different definitions for literature and children! Before the 19th century, very few books were specifically aimed at children. Since then, changing attitudes towards childhood and children's development, together with the increasing sophistication of printing technology, have led to the development of children's literature as a major industry. However, there is no simple and straightforward definition of children's literature that can be applied with the same validity in different times and in different contexts. Just as the concepts of "child," "childhood," and "literature" have evolved over time, so have the definitions of "children's literature." It is not easy to define “childhood” or “literature”. Some authors argue that children's literature differs from adult literature only to a certain degree (Lukens, 1995); others (Lesnik-Oberstein, 1996) argue that it differs in terms of its nature, that is, the word "literature", when used in the context of "children's literature", cannot necessarily be directly related to the word “literature” as it is used. in other contexts. So, for example, Bottigheimer argues that children's literature is "an important system in itself." To further complicate matters, some argue that to be included in the category of "children's literature", the writing must be of "good quality". So, for example, Hillman would exclude from the category of “children’s literature” writings that are “heavy,” “too predictable,” or “too illogical.” Precisely whether a work meets these extremely vague criteria remains largely an open question. Definitions of children's literature can be classified into three main categories. Townsend observes that "any boundary drawn to confine children and their books to their own special corner is artificial", and therefore that "[the] only practical definition of a children's book today – absurd as that may seem – is “a book that appears on a publisher’s children’s list”. Besides the fact that once again the word “book” appears in this definition, its usefulness is questionable. This definition would exclude works that appear in electronic format and are not included in publishers' catalogs. It would, however, include books designed for adults and adapted for children. In this regard, this may align with the views of Weinreich and Bartlett who include in their definition of children's literature those books originally written for adults and which have been reworked with children in mind..