His justification of this is his subjective critique of knowledge. He states that we have in our minds evaluations of things prior to our thought. We are of an opinion of something due to our history, morals, upbringing, and culture and that opinion shapes the thought and forms an evaluation so nothing can be judged as it truly is. Also that are desire for knowledge is clouded because we want the thought and knowledge to help shape and makeup what we want it to. It is a form of the will to power. You take something and say you know it, your knowing it though is influenced by your personal thoughts and by the way you want it to affect the world and others around you. .
Nietzsche then goes on to ask the question of why do we want knowledge in the first place. His answer is that knowledge is a tool of the will to power. It is a tool to control the weak, we say we know something and it is right so you must do this. His second response would be that it is a reaction to the abyss of not knowing anything. We make up things and call it knowledge to help us understand the world around us. It is along the same lines as "man is the measure of all things." We "men" put words and meanings on everything to make us feel like we "know" what is going on. .
For Nietzsche, there is no objective order or structure in the world except what we give it. The nihilist and Nietzsche discover that all values are baseless and that reason is ineffective. "Every belief, every considering something-true," Nietzsche writes, "is necessarily false because there is simply no true world" (Will to Power). For Nietzsche I think, nihilism requires a radical denial of all imposed values and meaning: "Nihilism is not only the belief that everything deserves to perish; but one actually puts one's shoulder to the plough; one destroys" (Will to Power).
This view from Nietzsche is an exact opposite of that of Plato and his Theory of the Forms.