The fact that we can consult with our own spirit is a proof of the immortality of the spirit

There is another indication that we might mention concerning the immortality of the spirit. When man is confronted with a difficulty he seems to consult himself, as though there were a something within him, a power, a voice. He says, shall I do this, or shall I not do it? What shall be the consequence? What harm will result? Then from that consultant emanates a certain opinion, and that still small voice says that the results will be either favorable or unfavorable, and advises that the matter should or should not be carried out. What is that something man consults when he thus deliberates? It is undoubtedly the spirit, which is changeless and therefore immortal. 
(‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)