Not conjecture. Reason and experience. Virtue is a successful behavior. The absence of virtue is a failed behavior.
Your reasoning is limited to human capacity, as is your experience ~ and so you cannot use merely your reason and experience to establish that virtues are "universal" to anything other than humans....rendering what youre saying conjecture, as I postulated.
You defined virtue as a behaviour leading to success...
then when asked to define success....you said it is virtue.
Strong circular argument, batman.