The Buddha teaches that there is one defilement which gives rise to all the others, one root which holds them all in place. This root is ignorance (avijja). Ignorance is not mere absence of knowledge, a lack of knowing particular pieces of information. Ignorance can co-exist with a vast accumulation of itemized knowledge, and in its own way it can be tremendously shrewd and resourceful. As the basic root of dukkha, ignorance is a fundamental darkness shrouding the mind. Sometimes this ignorance operates in a passive manner, merely obscuring correct understanding. At other times it takes on an active role: it becomes the great deceiver, conjuring up a mass of distorted perceptions and conceptions which the mind grasps as attributes of the world, unaware they are its own deluded constructs.
~Bhikkhu Bodhi, in The Noble Eightfold Path