The party who asserts a claim must prove it. You are not obliged to prove a negative or to disprove a fiction launched without evidence.
In lawful process, an accuser shows an injured party, facts under oath, and a causal link to harm. In commercial process, a claimant shows a contract, accounts, and authority. Until then, the burden stays with them.
People lose by volunteering explanations, arguing details, and accepting the frame of the claim. Each defensive speech can be treated as joinder and as shifting the burden onto you. Prefer verification requests, jurisdictional challenges, and silence after rights are reserved.
“Who is the claimant? Who is the injured party? Where is the contract? Show evidence under full commercial liability.” Then wait. The burden is not yours to carry.