Before you start setting tile can you use mortar as a thin set?

Before you start setting tile can you use mortar as a thin set?
The question is not clear in respect of scenario. However, the basethe tile is to be fixed should be level and sound. The fixativeshould be correct for the environment the tile is to be used ini.e. water proof in a bathroom. The surface for fixing to should beof a nature that allows the fixative to adhere correctly. Theinformation for this is supplied by the manufacturer.