Kosher ingredients

Question

I am new to this web site and have read through many of the teshuvos that the rav has posted. I have noticed that the Rav is of the opinion that a product can be determined to be kosher based on the ingredients alone (without an explicit hechsher). This approach appears logical but seems to go against conventional wisdom. I am curious why this is not the mainstream view. Also I would love to bring this up with some of my frum friends but they are always asking for sources. Can you point me in the direction of a source that supports the "ingredient kosher" perspective. Much appreciated, jf