What does it mean that supporting relationships build confidence?

Ideas supported by multiple independent lines of evidence are more reliable.

Example: Continental drift was proposed by Alfred Wegener in 1912, but for decades the geological establishment dismissed it — largely because Wegener could not explain the mechanism by which continents moved. He had one line of evidence: the shapes of the continents fit together like puzzle pieces. That alone was not compelling. Then, over the following decades, independent lines of evidence accumulated. Paleontologists found identical fossil species on continents separated by thousands of miles of ocean. Geophysicists discovered matching magnetic stripe patterns on either side of mid-ocean ridges. Seismologists mapped earthquake epicenters and found they traced narrow, continuous bands that outlined the edges of moving plates. Geologists found identical rock formations and mineral deposits on coastlines that had once been joined. No single line of evidence was conclusive. But when five independent disciplines — each using different methods, different instruments, different assumptions — all converged on the same conclusion, the theory of plate tectonics became one of the most robust in all of science. The supporting relationships between those lines of evidence did not merely add up. They multiplied. Each new independent source of support made the conclusion not incrementally more likely, but dramatically more certain.

Try this: Pick one belief you hold with high confidence — a belief about your health, your career, your relationships, or how some system works. Write it as a single declarative sentence. Now list every independent line of evidence that supports it. Be rigorous: each line must come from a genuinely different source or method. Observation counts as one line. A friend's testimony counts as another — but only if they arrived at their view independently, not by hearing it from you. Data you collected counts as another, but only if the measurement method differs from your observation. Published research counts, but only if you actually read it and it addresses your specific claim. Now count: how many truly independent lines of support do you have? If the answer is one or two, your confidence may be higher than the evidence warrants. If the answer is four or more from genuinely independent sources, you have identified a belief with robust evidential support. Finally, for a belief where you found only one or two lines, identify one specific action you could take to seek an independent additional source of evidence.