No amount of stimuli, experience, or thinking is sufficient to prove the existence of an external agent. Despite the previous statements, there is no essential conflict between science and constructivism at the operational level. In fact, scientists readily admit that all we can ever do is construct a model of external reality, assuming it exists. Thus, all that we know is actually a set of stimuli and experiences. This is totally in accord with the scientific view.
Prior knowledge impacts the learning process.
Initial understanding is local, not global. Building useful knowledge structures requires effortful and purposeful activity. The constructivist perspective is clearly divergent from earlier views of education that presumed we could put or pour information directly into a student's head. Starting from constructivism, real learning can occur only when the learner is actively engaged in operating on, or mentally processing, incoming stimuli. Furthermore, the interpretation of stimuli depends upon previously constructed learning.