It's not unusual to have sources which can be reliably dated to a period but which cannot be attributed to a named individual. These sources become, therefore, anonymous (no-named) and need to be treated much more carefully as it can be difficult to contextualise the author's life before creating the source. They are not inherently useless just because we can't slap a name on the opinions, but using these types of source does require great nuance. So while the source author is anonymous the source itself should be findable and readable.