The accounting theory would be Objectivity principle. The definition for Objectivity would be that accounting information and financial reporting should be independent and supported with unbiased evidence.

For transactions to be reliable and unbiased, it should be given by a third party. For example, to record a purchase of Furniture, a copy of supplier invoice and goods receipt note / delivery order should be sighted and acknowledged that items are fully received. By doing this, goods purchased is verified by invoice provided by vendor/supplier and not an internal document produced by the own company. If no source documents (invoice/delivery order) are available, it is unable to confirm if the purchase actually went through and thus, entries passed would not be reliable upon.