The nature of knowledge and its validity must always be challenged. It is known that humans have very few answers to questions that are perfect in quality or nature. Even if we come close to being absolute, we may only have part of the answer.
When science answers a question, there is always room for another answer or set of answers. We only know part of what we could know and we only understand part of what is. If we did not challenge modernity then we would not have the opportunity to inch closer to absolute truth. The post modernist might say that it is difficult to be objective and that environmental influences will interfere with our quality or state of being. This interference may alter the approach to whatever scientific question we seek to answer.
If we do not question the validity of the source then the nature of the initial knowledge may be skewed by the perspective of the individual gathering or looking into this knowledge.