October 30

Materialists say, ‘Where is the soul? What is it? We cannot see it, neither can we touch it’. This is how we must answer them: However much the mineral may progress, it cannot comprehend the vegetable world. Now, that lack of comprehension does not prove the non-existence of the plant! To however great a degree the plant may have evolved, it is unable to understand the animal world; this ignorance is no proof that the animal does not exist! The animal, be he never so highly developed, cannot imagine the intelligence of man, neither can he realize the nature of his soul. But, again, this does not prove that man is without intellect, or without soul. It only demonstrates this, that one form of existence is incapable of comprehending a form superior to itself. This flower may be unconscious of such a being as man, but the fact of its ignorance does not prevent the existence of humanity.

In the same way, if materialists do not believe in the existence of the soul, their unbelief does not prove that there is no such realm as the world of spirit. 
- ‘Abdu’l-Baha  (‘Paris Talks)