See the "Mary's room" thought experiment. Thinking and reasoning about all the measurable properties of a phenomenon is way different than experiencing them. This doesn't necessarily mean that subjective perceptions have an immaterial existence, but it provides an approach to analyzing the mind that can't be achieved by physical measurement alone.
A better question would be "how can you try to understand consciousness without instrospection?" Studying consciousness merely by performing brain scans and electroencephalograms, without asking the subject what she's experiencing, would surely provide a poor and incomplete perspective.
