[kosta50]

Are you confusing dependent in existence with dependent in the syllogism?

Aquinas tied cause with existence by saying that all that exists has a cause other than itself. In that case, if the first cause were to exist, it would also have to have a cause.

[D-fendr]

all that exists has a cause other than itself

I believe that would be a misstatement. He argued that secondary, dependent, causes are all around us and they could not exist if there were not a first efficient cause.