It has been rightly said that all people are not alike. They differ considerably. In no respect is this truer than in the field of sensations. Different people react differently to sensations. Even the same sensations or similar sensations do not evoke the same or similar reactions. This is because some people in this world are wise and some otherwise. Those who are wise understand sensations while others do not.
Difference in Reactions
To understand this difference in reaction we must realize what sensations are and how thorough understanding accompanied by wisdom affects them.
Sensation according to the Western interpretation is the 'operation of the senses'. It is the 'function of the senses'. In fact, it is a 'mental condition or physical feeling resulting from the stimulation of a sense organ'. Sensations produce a kind of impression on one's body and mind. This impression gives rise to a certain condition or feeling.
This interpretation is a generalised perception. The Pali texts offer a more lucid explanation. In Samyutta Nikaya,1 sensations have been compared to winds blowing from different directions and these winds are sometimes dust-laden or dustless, sometimes hot or cold and sometimes they take the form of fierce gales or gentle breezes. These sensations according to the texts are of three types. They are pleasant, unpleasant or neutral.
These three types of sensations give rise to feelings of happiness, that is, attachment (raga), feelings of aversion, that is, hatred (dosa), and feelings of confusion, that is, ignorance (moha), according to the category to which they belong.
Raga - Dosa - Moha-Root Cause of Misery
The Buddha knew for certain that infatuated by ignorance, beings go to bad states (duggati) and he stated this in Itivuttaka while addressing the monks, 'Bhikkhus, leave one thing, give up this one thing (ignorance) and I shall stand guarantee for your non-return to this world.' He emphatically added that the only way to destroy this ignorance was to comprehend it thoroughly (tam moham sammadannaya pajahanti vipassino). He asserted the same thing about hatred (dosa) and attachment (raga).
However, one cannot eradicate raga, dosa, and moha unless one has a method to understand the true nature of how they arise, make a person happy or unhappy, and their characteristics.
Arising of Raga, Dosa and Moha
A common man who does not know the true nature of sensations gets attached to pleasant sensations and, thinking that they are permanent, develops raga. The same is true of dosa and moha. These three in their turn give rise to dukkha. The precursor of these sensations is contact (phassa).
Seven characteristics of these sensations
One must know the true dhamma of these sensations. According to the Samyutta Nikaya they are seven:
1. impermanent (anicca)
2. compounded (sankhata)
3. arising owing to a cause (paticcasamuppada)
4. perishable by nature (khaya dhamma)
5. passing away by nature (vaya dhamma)
6. detached by nature (viraga dhamma)
7. ceasing by nature (nirodha dhamma)
Sensations: their relation to the Noble Truths
The Pali texts, besides expounding the true nature of sensations and how they bring about dukkha, delve deeper into this phenomenon. In the Anguttara Nikaya,2 there is a detailed explanation of how the experience of bodily sensations is related to The Four Noble Truths-
And for one who is actually experiencing bodily sensations, O meditators,
I teach, I expound the Truth-'This is suffering.'
I teach, I expound the Truth-'This is the arising of suffering.'
I teach, I expound the Truth-'This is the cessation of suffering.'
I teach, I expound the Truth-'This is the path leading to the cessation of suffering.'
The Nature of Sampajanna and its Role in Bringing about the Cessation of Suffering
The third and the most important and crucial of all the Noble Truths takes place when sampajanna plays the key role. But what is sampajanna?
Sampajanna is understanding thoroughly with wisdom. It is a gradual process and consists of five steps.

Five Steps of Sampajanna

First, the meditator must know consciously what is happening to him. He must be conscious and careful about the arising of sensations and the resultant vices-raga, dosa and moha.
Second, this consciousness must help establish him firmly from moment to moment and create a special kind of awareness in him.
Third, this awareness must be developed to such an extent that sensations become a matter of mere observation and perfect understanding. They no longer create an impression on the body or the mind.
Fourth, having reached this state, the meditator must learn to dwell in that state of heightened prolonged awareness, where the meditator no longer grasps anything. In fact, there is nothing for him to grasp. This is not mere knowledge (as is the case with most of us) but actual realisation of that state of non-grasping.
Fifth, by dwelling in this state of non-grasping for a prolonged period, the meditator experiences the cessation of experience.
These five steps, taken firmly, consciously and confidently teach the meditator to step aside and stand apart from what he experiences. Therefore, he does not allow sensations to create any impression on him and becomes immune to all the after-effects of sensations.
Sampajanna Must be Accompanied by Exemplary Alertness
This however demands exemplary alertness on his part. It is said in the Sampajana Pabbam of the Mahasatipatthana Sutta, 'One who has the highest goal in his mind, should always and forever be watchful and be possessed of thorough understanding with wisdom on all occasions, in all situations, while looking straight ahead and sideways, while bending and stretching, while wearing robes and carrying a bowl, while chewing, drinking and savouring and even while attending the calls of nature, while walking, standing, sitting, lying down, sleeping, waking, speaking and remaining silent.'
This shows that watchfulness on the one hand and the possession of thorough understanding with wisdom on the other are both needed to attain liberation. Indeed, this sampajanna is the meditator's armour, a jacket not just bullet-proof, but sorrow-proof, craving-proof, indulgence-proof.
This is the most precious gift that the Buddha's teaching gives us. The time-clock of Vipassana has struck and the technique is again being spread by Shri S. N. Goenka, and before him, by his teacher Sayagyi U Ba Khin.

Notes: [References from VRI edition in brackets]

1. Samyutta Nikaya, Vedana Samyutta, PTS 26.12.12 [VRI 2.4.249-268]
2. Anguttara Nikaya PTS 3.7.1 [VRI 1.3.62]