The tests of truth are logical consistency, agreement with experience and economy of explanation. But one can gain a greater understanding of how to distinguish between ones own opinions and knowledge, and what lies within ones own control, with Zeno of Citium’s theory of knowledge, which he taught using his hands to explain the various stages of acquiring knowledge:
Stage 1: Hand held wide open = IMPRESSIONS
Stage 2: Hand closes to form fist = ASSENT TO CONVICTION
Stage 3: Other hand grasps fist = KNOWLEDGE
Here is my somewhat modified version of Zeno’s Theory of Knowledge:
Stage 1: IMPRESSIONS: Everything we can possibly know is initially derived from impressions made on our senses—i.e. hearing, seeing, touching, tasting and smelling. It is from these impressions our memories are built, which we recall as mental images, or appearances in the mind. In terms of memory, a newly born baby’s mind can be likened to a clean sheet of paper, ready to be written on.
Stage 2: ASSENT TO CONVICTION: This is where our reasoning faculty forms general notions, through recognising relationships and similarities between the impressions. But impressions have varying degrees of clarity. Some, such as good and bad, are strong and demand immediate assent, whereas others require deliberate reflection, and the notions formed from weak indistinct impressions—although they may be true—are at this stage, merely our own formed Opinions or Beliefs.
Stage 3: KNOWLEDGE: The impressions from which genuine knowledge is formed are clear and precise, and this knowledge (deductions, conclusions, understanding) cannot be removed, and can be confirmed and reinforced by further impressions. Kataleptike Phantasia is the ancient Greek name for the impressions that get a firm grip on reality—In other words, the numerous events you can clearly recall that have resulted in genuine knowledge.