Philip Lipton is the son of Dwight K. Schrute and Angela Martin. Philip is named after one of Angela's cats, sparking a debate with Pam Halpert, who chose the name Phillip for her then-unborn son as a tribute to her grandfather.
Initially, Dwight believes that Philip is his own son, and in "Free Family Portrait Studio", he steals one of Philip's diapers and takes it for paternity testing. The results of the test eventually proved that Philip is, in fact, not Dwight's son. In "A.A.R.M." Angela reveals to Dwight that Philip is indeed his son. It is currently unclear how Angela was able to make Dwight believe that he was not the father after paternity testing, but, considering the fact that Jim and Pam's children were also in the office at the time that Dwight stole the diaper, it is likely that he simply brought the wrong diaper for testing.