Final answer:
Synthetic a priori statements are principles that guide research and are foundational assumptions used in reasoning and science, understood through reasoning rather than experience.
Step-by-step explanation:
The best description of synthetic a priori statements is that they are principles that guide research and can be considered underlying rules that all natural processes follow. These statements are not derived from empirical evidence but through reasoning, even though they can sometimes be informed by experience. An example of synthetic a priori knowledge might be the principle of causality, which states that every event has a cause. This principle is not derived from experience, as we cannot observe every event, but it is a foundational assumption in both everyday reasoning and scientific investigation.