This is from Bialik and Ravnitsky's introduction to Sefer Ha-Aggadah. The introduction was not translated for the English edition, so this is my translation, which does not even attempt to emulate Bialik's linguistic virtuosity. Those who can read it in Hebrew are encouraged to do so.
Aggadah is the main literary form that dominated for centuries in the world of free expression, national and personal, of the nation of Israel.
According to its nature, aggadah is not a temporary and passing literary vision, but rather it is a classic creation of the spirit of our people, a creation that has fruits for its time and whose principal endures for the generations. At its essence and in general it is one of the great revelations of the spirit of the people and its individuals. Many generations and persons have sunk into it, knowingly or unknowingly, the power of their excellent creation and all the richness of their spirit; many generations have cared for building it and improving it until it became an entire world unto itself, an amazing and unique world, with its own grace and beauty. And a creation like this -- it is impossible for it not to have much from the eternal and the universal, which must remain existing as it is, in its image and in its likeness, as an exemplary creation for the generations of the world.
As far as its content -- aggadah includes many subjects from the views of the people and its individuals on the nation of Israel and its national holdings, on its eternal life and its temporal life, on its events and its future, on the great ones of the nation, their happenings and deeds, their ways and traits, on humanity and the world, on beliefs and faith, on the wisdom of life and on "the mystery of the world" (or "the eternal questions", in contemporary language), on "the kingdom of earth" and "the kingdom of heaven", etc. etc. -- all said, there is no field of feeling and thought, from those, of course, reached by the power of knowledge and imagination in those days -- on which aggadah does not touch in some way.
As far as its form -- almost all of the literary genres that were used in Israel in the aforementioned fields in those days made their way into aggadah: stories of deeds and conversations, real and imaginary descriptions, homilies and sayings, proverbs and clarifications, words of poetry and song, embellishments, words of [?] and words of jokes, language of wisdom and riddles, etc. etc.
And the linguistic style of aggadah has also arrived -- in selected examples, of course -- at a great fullness. This style is clear and simple, easy and flowing, comfortable and spacious, having much of the constancy and exactness of developed literary language, much -- from the pent up and faithful internal warmth of the language of the heart, much from the picturesqueness and exaggeration of the power of the popular imagination, and much from the terseness and sharpness of the language of the sages and popular proverbs. In some of the most ancient divisions of aggadah (mishnayot and baraitot) much of the essential vitality and refreshed health of the real language of the people is preserved, a language from which the "scent of the earth" has not yet faded, and whose natural connection between it and its people still stands and endures in a known measure.
One who wants, then, to get to know the nation of Israel from the aforementioned sides -- has no choice but to "go to the aggadah", this literary field that is great in quantity and quality, multifaceted and heterogeneous, which is a unique division of the people of Israel, and for many centuries after the "sealing of vision", the spirit of our people and its creative power have been clothed in it in such a unique way and in an original way with no model in subsequent generations.
Through aggadah one enters the house of the complete life of the nation of Israel and looks at its "innermost". One comes to know the sovereign of the nation and peeks into its world as it is, its customs and its character, its unique light and atmosphere, as it is built and improved in the heart of the whole nation, from its small ones to its great ones. Because aggadah in its entirety is not the inheritance of individuals, but is a shared creation of the whole nation, and all are partners in the creations of its world.