First of all, the principle must be disclosed by the agent to the third. For example in the case of Keighley, the agent has purchased the wheat at a higher price in the agent’s own name. Therefore, it render the principal an undisclosed principal and has no right in ratifying the contract.
Secondly, the principal must be inexistent at the time o the agent act. In the case of Kelner v baxter, a promoter had entered into a contract on behalf of a company which is not yet to be formed. The court held that the promoter is liable to the third party.
Thirdly, the principal must have the contracted capacity at the time when the act was done. The principal must have to capacity to make a contract when the agent is making a contract. For example in the case of Boston Deep Sea Fishing Co, the principal was an enemy alien when the contract was made by his agent. Therefore, as an enemy, such capacity is not recognized by the law.
Lastly, an act which is void cannot be ratified. Is pretty much straight forward that something which is void does not has legal effect anymore. Basically, there is no need in to further ratify.