Critical thinking is one thing but that "thing" is a process rather than an individual item. Critical thinking, like science, is a process that iterates upon itself. You analyze the information you have and make a conclusion based on that information. The "critical" part comes in when you take a step back and then use that same process to analyze your conclusion and poke holes in it, checking to see if the information you have supports that hole or supports the conclusion.

It's like saying that a computer is one thing despite the fact that the one thing is made up of multiple pieces.