An affidavit must be made based on the personal knowledge of the affiant (the person who makes an affidavit). The affiant must state in his/her affidavit how he/she had knowledge of the facts. If the affiant fails to provide such evidence, then the affidavit will be presumed to have been made without personal knowledge.
However, when it is very obvious that an affidavit is based on the personal knowledge of the affiant, there is no requirement that the affiant show how he/she had personal knowledge of the facts. Similarly, when it appears from a document that the affidavit is based upon the personal knowledge of the affiant, then also the affiant need not furnish additional evidence to show that he/she had personal knowledge.
Opinions or beliefs of an expert based on his/her special training and experience are not considered to be of personal knowledge for the purpose of an affidavit.