What is Siddhartha's inner voice? Is it the same as, or similiar to, his conscience?

I think the two are similar. In chapter seven Siddhartha aquires wealth and status. This ignites desire within him, He wants more and more and is trapped by this endless cycle of want. One night Siddhartha dreams about a songbird that belongs to Kamala. He finds the dead bird and trows it away into the street. He realizes he has thrown away goodness. Now his inner voice talks to him and tells him he must sever the cycle of desire before it is too late.