(Exodus 22:9) says "no one seeing" in the context of a guardian who claims an animal was stolen from his care. The Mekhilta explains: "no one seeing" means no witnesses were present. If there are no witnesses to confirm or deny the guardian's story, the matter proceeds to the next stage.
(Exodus 22:10): "The oath of the Lord shall be between the two of them — that he did not send his hand against the deposit of his neighbor." In the absence of witnesses, the guardian must swear a sacred oath that he did not misappropriate the animal himself. The oath substitutes for the testimony that witnesses would have provided.
But if witnesses are available — if someone actually saw what happened to the animal — the guardian is exempt from the oath entirely. He does not need to swear because the facts can be established through testimony.
This passage establishes the relationship between oaths and evidence in Jewish law. The oath is not the preferred mechanism for resolving disputes. It is the fallback when direct evidence is unavailable. Witness testimony always takes priority over swearing. The guardian swears only because there is nothing better. When there is something better — an eyewitness — the oath becomes unnecessary. The system prefers objective evidence over subjective declarations, using the oath as a last resort rather than a standard procedure.