In the Western scientific view, the definition of mind is not very clear. Mind is often thought of as an effect of the physical brain in which the neurons carry out their chemical-electrical actions and a sort of illusion of consciousness and of mind results. From this perspective, minds differ mainly according to brain size and the quantity of neuronal fibers and synapses in the brain.
The Yoga view of mind is completely different. In Yoga philosophy, the mind is viewed as a subtle energy field, somewhat like a magnetic-energy field, rather than a physical object. This field of energy can be stronger or weaker, and it connects with the physical world with the help of the physical brain.
Although the mind in the Yoga conception is an energy field, it is still considered material and the character of any particular mind depends upon which of three energetic constituents is predominant in the makeup of that mind.
According to Yoga philosophy and its sister philosophy known as Samkya, everything in the world is formed out of a single material substance called prakṛti, which is composed of a mix of three energetic constituents known as the gunas. So gross objects, such as bodies, rocks, and liquids, as well as subtle objects, such as prana, the subtle energy that enlivens the body, and the even subtler energy field of mind, are composed of the gunas.
There are three gunas; they are called sattva, rajas, and tamas and each of them exist in every material thing in varying proportions.
Each guna provides something needed for objects to exist. Tamas provides inertia; rajas provides dynamism; and sattva provides illumination.
Without tamas, things would not hold together; they would not be solid or stable. Without rajas, objects would not move; they would be inert. Without sattva, there would be no illumination, no lightness, no enjoyment.
One object differs from another object due to the relative proportion of each guna within it. So, a material object containing so much sattva, so much rajas and so much tamas, might be a clod of dirt, while a different proportion of each guna may result in a shining gemstone.
Furthermore, whichever guna is in the greatest proportion in the makeup of any particular object lends its quality to the object. So, things in which sattva predominates are illumined, like light or wisdom; things in which rajas predominates move, like the blowing wind; and things in which tamas predominates are unmoving, dark, and heavy, like a clod of dirt.
As was mentioned, since mind is material, it too is composed of the gunas, and the guna that is predominant in any particular mind determines the character of that mind.
So, a mind in which tamas predominates is filled with inertia, is inflexible, slothful, and slow. Such minds are said to be obstinate, inattentive, and to expect others to fulfill their desires. Minds in which rajas predominates are dynamic, quick, and creative. Full of desires, attachments, and aversions, rajasic minds love to compete and to win. Sattvic minds are illumined, joyous, serene and gentle. They are drawn to Truth, wisdom and service to humanity.
From this brief discussion, you can see that the gunas greatly affect the sort of mind you have. By looking at your tendencies, you can type your mind as being predominately tamasic, rajasic, or sattvic.
The nature of a person’s mind will greatly affect how s/he acts and thinks and the sort of person s/he appears to be. To some extent, these qualities are set, and to an extent they can change as a result of the actions of that person.
So, for example, by practicing being more active and disciplined, a person with a tamasic mind may be able to move his mind toward a more rajasic character. It’s not easy to do this, but to some extent it can be done. An example might be the person who, previously depressed and unproductive due to chemical dependency, is able to overcome the addiction and become active and productive.
Similarly, a person with a predominately rajasic mind, by focusing on deepening and concentrating awareness, charity, and wisdom, may be able to bring more sattva into his/her mind. So a person who is focused on winning the game of life and on sensual enjoyment, may focus on methods to gain inner-awareness and peace and, over time, his/her mind may become more sattvic. Actually, most Yoga practices do just this because reaching the heights of Yoga requires a predominately sattvic mind.
An understanding of the three gunas and their effects on the mind can help a person who is willing to do the work to transform the character of his/her mind. This is especially so for yoga practitioners who want to reach deeper levels of meditation, for only a very sattvic mind is calm and clear enough to enter the deepest states of meditation.