The scriptures mention a few occasions when the Buddha remained silent to questions posed to Him.
The Buddha who had truly realized the nature of these issues observed noble silence. An ordinary person who is still unenlightened might have a lot to say, but all of it would be sheer conjecture based on his imagination. When the Buddha knew that the questioner was not in a position to understand the answer to the question because of its profundity,or the real significance of the answer to his question or if the questions themselves were wrongly put in the first place, the Blessed One remained silent. Silence itself is a statement. On several occasions the Buddha did not respond to these metaphysical and speculative questions and there are reasons behind His noble silence.
Some scholars, owing to their misunderstanding of the Buddha's silence, came to the hasty conclusion that the Buddha was unable to answer to these questions. According to the Buddha, there are several ways of answering various types of questions.
The first type of question is one that requires a definite answer, such as a 'yes' or 'no'.
The second type of question is one requiring an analytical answer. We need to analyze what conditions make it possible for a murderer to become Buddha within one lifetime otherwise, it will not be answered correctly and comprehensively.
The third type of question is one where it is necessary to ask a counter question to help the questioner to think through. If you ask, "Why is it wrong to kill or hurt other living beings?' the core of the question is, 'How does it feel when others try to kill or hurt you?' Reply then comes by implication.
The fourth type of question is one that should be dropped. It means that you are liable to not to answer it. These are the questions which are speculative in nature, and any answer to such questions may, (though not necessarily, otherwise all the discoveries through theoretical sciences might not have been possible), only create confusion. Buddhism uses as example a question , 'Does the universe have a beginning or not?' People can discuss such questions for years without coming to a conclusion. They can only answer such questions based on their imagination, not on real understanding.
The paucity of our human vocabulary which is built upon relative experiences cannot hope to convey the depth and dimensions of Reality which a person has not experienced himself through Insight. On several occasions, the Buddha had very patiently explained that human language was too limited and could not describe the Ultimate Truth. If the Ultimate Truth is absolute, then it does not have any point of reference for worldliness with only mundane experiences and relative understanding to fully comprehend it. When people try to do so with their limited mental conception, they misunderstand the Truth. The listener who had not realized the Truth could not fathom the explanation given.
Some of the questions to which the Buddha remained silent are as following:
Two basic views
The view of existence as professed in the Brahmainsm;
The propositions that the world is eternal, that the world is infinite, that the Tathagatha exists after death, and that the Self is independent of the body reflect the view of existence.
According to Buddha, the eternalists view this Self as permanent and unchanging. When the body dies, this Self will not die because the Self is by nature unchanging. If that is the case, it does not matter what this body does, actions of the body will not affect the destiny of the Self. This view is incompatible with moral responsibility because if the Self is eternal and unchanging, it will not be affected by wholesome and unwholesome actions. If he says "I," the Self is eternal, people would fall to the position of eternalism.
The view of nonexistence the view of the Materialists and Hedonists
The propositions that the world is not eternal, that the world is finite, that the Tathagata does not exist after death, and that the self is identical with the body reflect the view of nonexistence
According to Buddha, if the self were identical with the body and the self dies along with the body, then it does not matter what the body does. If you believe that existence ends at death, there will be no necessary constraint upon action. But in a situation where things exist through interdependent origination, absolute existence and nonexistence are impossible.
If Buddha says they are not eternal, they would fall to the position of nihilism, since they would not understand his answer, it was,therefore, more skillful not to specify an answer at all.
According to Buddhist teachings, when ignorance, craving, and clinging are present, the world exists; when they are not present, the world ceases to exist. Hence the question of the absolute existence or nonexistence of the world is unanswerable. Existence and nonexistence, taken as absolute ideas, do not apply to things as they really are. This is why the Buddha refuses to agree to absolute statements about the nature of things. According to Buddhist teachings, Buddha saw that the absolute categories of metaphysics do not apply to things as they really are.
The (Hinayana)Theravada Version
Ten unspecified points appears in the Pali canon (Pali: Cula-Malunkyovada Sutta, within the The Collection of Middle-Length Discourses (Pali: Majjhima Nikaya).
In order to turn ,the monk Malunkyaputta, distracted by metaphysical speculation during his meditation, back to his intensive meditation practice,Buddha remained silent when he asked whether:
the universe is eternal,
the universe is not eternal,
the universe is finite,
the universe is infinite,
after death, a Buddha continues to exist,
after death, a Buddha does not continue to exist,
after death, a Buddha both continues to exist and not to exist,
after death, a Buddha neither continues to exist or not to exist,
the body and the “self” are the same entity,
the body and the “self” are totally separate and different entities.
Questions referring to the world: concerning the existence of the world in time
Is the world eternal?
Pali texts omit "both" and "neither".
Questions referring to the world: concerning the existence of the world in space
Is the world finite?
Pali texts omit "both" and "neither".
Questions referring to personal experience
Is the self identical with the body?
or is it different from the body?
Questions referring to life after death
Does the Tathagata exist after death?
The Mahayana Version
Buddha did not answer when they asked, are the "I" or the "self :
not eternal, since they undergo gross impermanence at the time of their destruction?
as both, in the sense that some beings and their environments, like the Creator Brahma and his heaven, are eternal; while all else, such as his creations, are not eternal and end at the time of their destruction,
neither, since it is impossible to know?
Are “I’s” or “selves” and the universe:
both finite and infinite, in the sense that limited beings (sentient beings) are infinite in number, but the universe is finite in size?
neither, since it is impossible to know?
Does the “I” or the “self” of a Buddha:
continue to exist after death?
not continue after death?
both, in the sense that the body does not continue, but the life-force does,
the body and the "self" are the same entity?
totally separate and different entities?
The Buddha did maintain silence to all these questions put to Him. He was under no obligation to respond to questions which reflected gross misunderstanding on the part of spiritual development. He was a practical Teacher, full of compassion and wisdom. He always spoke to people fully understanding their temperament, capability and capacity to comprehend. Questions were answered to help a person towards self-realization, not as a way of showing His towering wisdom.