Demonstrative and possessive adjectives are determiners. Since a noun phrase takes only one determiner, do not use an article if you modify the noun with a demonstrative adjective or a possessive adjective.
The purpose of the this
thesis . . .
The purpose of this the
thesis . . .
The purpose of this
thesis . . .
The purpose of the my
thesis . . .
The purpose of my the
thesis . . .
The purpose of my
thesis . . .