If properties can stand in for fields, why do we ever need fields?

Microsoft's discussion of them includes "A field defines a storage location... A property definition contains declarations for a get and set accessor that retrieves and assigns the value of that property". A discussion at stackoverflow seemed to imply that there's always a field lurking somewhere under the hood of a property. I had assumed maybe properties were a new feature, but they seem to have been part of c# 1. If a value can't be modified or is private with few rules about its access, then the field might be safe to access or you might not gain anything by writing a property. if properties are meant to protect a field and apply validation, a complete absence of rules or transformation would mean a property would add extra code without adding value. Apart from this, a couple people discussing this at https://stackoverflow.com/questions/2166433/why-ever-use-fields-instead-of-properties suggest that ref/out and readonly only worked on fields.