The novel is the story of a frustration.
Andrés Hurtado, his protagonist, seeks explanation absurd and unjust reality. And it is no mere intellectual curiosity that drives him, but the need to get some "practical truths" with which he conducted himself in life.
The attempt fails because the approach is too radical, reason and life are seen as irreconcilable principles. In the struggle between the tree of knowledge and the tree of life, Andrew will give in to the latter, which show strong, indomitable. Life as a "blind force" that is opposed to reason is a 'turbulent flow' which carries with its impetus to the individual, even if he wanted to avoid growing abstention. Thus, the noble efforts of Andrew for understanding reality and find Instead it will be a wasted effort, finally settled in tragedy.
Baroja projects his own ideas, we must not forget the high-autobiographical tale, based, in this case, where the favorite philosophers: Kant and Schopenhauer. It is also observed at some point the influence of Nietzsche.
The vicissitudes of the protagonist has a symbolic value. In the last chapter of the novel - I had some precursor - is revealed that Andrew's death is symbolic of an idealistic attitude, beaten in their opener against reality, Andrew, however, gives hope to a future in which science and justice will get rid the man of her servitude. Again, we noted the autobiographical component of the story, as Baroja, at a time of personal growth and creative play, in large part, what were their own disappointments of youth.
In the background, the novel is a sociological document, political and cultural end of the century Spain. That reality crashing against Andrés Hurtado is given precisely by the wide range of defects, flaws and gaps that characterize, according to Baron, the national life around the key date of 1898. Andrés relations with the urban environment - Madrid - and the rural world - Alcolea del Campo - allows the novelist to present an overview of the Spanish situation of the moment. The tone is extremely critical: the lack of solidarity is social, corruption and inefficiency of the political system, the stagnation of science, environmental or cultural provincialism, Baroja's complaint is relentless.
And finally, be added that the novel is a clear indication of noventayochista literature. A novel as the tree of knowledge could only occur in the context of the crisis of the Century: Baroja raises pessimism and 'disappointment' own 98 to the category of existential pessimism and disappointment. But the problem has only existential character or the so-called 'theme of Spain', in the pages of the novel, we find other issues common to the writers of the generation, such as apathy (boredom or boredom with things) or ataraxia (state of contemplation and abstinence for life.)
With regard to style and design of the novel, Baroja aims to create a new language, away from nineteenth century realism. Thus, although his work will ultimately be likened to that of Galdós presents a new kind of realism. If Galdós is objective, calm, realistic, Baroja is, however, subjective, passionate, expressionist / impressionist. No try to copy reality, but it appears filtered through the writer's self.
From the perspective of contemporary criticism, the tree of knowledge is characteristic of 98 work as far as it manifests the same radical ideology and the same inability to political action in most of the writers of the generation.