What essential trait is necessary for a legal contract to be valid?

For a legal contract to be valid, the essential trait required is the presence of bargaining power and consent from both parties. This means that both parties must willingly agree to the terms of the contract, demonstrating mutual understanding and acceptance. Without this mutual consent, a contract cannot be considered legally binding, as it lacks the necessary agreement that defines a contract's very nature.

While the other options might seem relevant, they are not fundamental requirements for the validity of a contract. Witness signatures or a notary public's approval can lend credibility and help with the enforcement of a contract, but they are not necessary for the contract to be considered valid in its essence. Similarly, having written documentation on parchment is not a strict requirement, as contracts can also be verbal (though it's advisable to have written contracts for clarity and evidence). Therefore, the core element that makes a contract valid is the agreement and consent of the parties involved.