it would be impossible to believe that one breathed without a belief in something that would be correctly termed lungs. You would have absolutely no concept of the it means to breathe. You would perceive feelings, but you would have no words or ideas on which to define those feelings as "breathing" hence it would be impossible to believe that you were breathing.
If breathing is the act of taking in oxygen to your body and exhaling it, and you believe you do that, you necessarily believe that there is some mechanism which facilitates that act, which are termed "lungs". What the lungs actually are is of no importance. You might believe that "lungs" are little dwarfs living in your stomach that open your nasal passages and invite air in. The idea would still be correctly defined as a mechanism by which we breathe, which humans have termed "lungs".
If breathing is the act of taking in oxygen to your body and exhaling it, and you believe you do that, you necessarily believe that there is some mechanism which facilitates that act, which are termed "lungs". What the lungs actually are is of no importance. You might believe that "lungs" are little dwarfs living in your stomach that open your nasal passages and invite air in. The idea would still be correctly defined as a mechanism by which we breathe, which humans have termed "lungs".