Many people who proclaim to be materialists are actually spiritual if the surface of their logic is scratched a bit.
Materialism can be defined as the position that there exists nothing but matter. Physical matter (including field, energy, etc) is the fundamental building block of this world. Everything that exists is a composition or product of matter. Everything in this world can be explained as the result of physical phenomena.
There is no individuality beyond the body. With the death of the body, my individuality is entirely gone without a trace. I simply cease to exist.
Thoughts are a product of the chemical and electrical actions happening in the brain. As this is an entirely physical system, the dynamics of every part of matter is the result of the dynamics that was set into motion during the Big Bang. This means, I have no control over my thoughts. Everything I think is the result of the dynamics of matter. There is no free will. I am destined by matter to think what I think and do what I do. So, I cannot be held responsible for what I think or what I do.
How many people who claim to be materialists accept this logical conclusion? Anyone who accepts free will cannot take materialism as the basis. Materialism and the concept of free will are contradictory.
Spiritualism assumes that I am a “self-willing” entity with an independent existence. The technical term for this entity with free will is “jiva”. The material world is called “jagat”.
The basic law of nature, the Law of Causation, which manifests in the physical world as the laws of conservation of various entities like matter, energy, momentum, charge, current, etc will apply to the jiva also. The jiva, which is free to decide what to do in a given situation, is responsible for the consequences of its action. The experiences that the jiva goes through are strictly the consequences of its own past actions, following the basic law of causation. The law of causation applied to the jiva is called the law of Karma.
Following the law of causation, everything the jiva faces has to be the result of its past action. One child is born in a very luxurious environment. Another child is born in a pathetic environment. So we have to assume past existence and actions of the jiva before birth. Similarly, people keep doing things till the end of the life of the body. Not all the results of all the actions are experiences in that life itself. So we have to assume a future existence after the drop of the body.
Now, which of these models is the truth? Just because materialism does not allow free will, can that be discarded as false? The choice between materialism and spiritualism does not come under science. It is not “falsifiable” (see http://en.wikipedia.org/wiki/Falsifiability). Only things that can be judged based on observations through physical means come under the purview of science. So no amount of observation and logic based on those observations can help us to choose between these two. Any choice is an unfounded speculation.
Where there is no approach by senses, the scriptures of religions come to help. The great philosopher Sankara says “ateendriyaarthe shaastram pramaanam” – The scriptures are the authority on matters which are not related to the physical world accessible to the senses. When we are left with no option other than mere speculation, religion comes in and gives a meaningful and fruitful choice, claiming the choice to be a divine revelation. In the world of the senses, science is authority. In the world beyond the senses, scriptures are the authority.
Advaita Vedanta considers this concept of jiva and jagat only as a relative model. This is only a “working model” to be used as a stepping stone to the absolute model. The absolute truth is that there is a more fundamental entity underlying both jiva and jagat. The nature of the entity is pure Consciousness. Discussion about that is beyond the scope of this article. You can read about that in articles like this.