Philo Judaeus was an ancient Jewish philosopher and theologian who lived at about the same time as Jesus. As a contemporary of Jesus and because he wrote a great deal about religion some have assumed that he certainly would have written about Jesus. But he did not, and that has been taken as evidence that Jesus did not exist, or if he did he was far less than what the gospels depict.
Is that a valid conclusion?
Here is what we know of Philo from his writings, most of it preserved by early Christians such as Eusebius. He was born in 20 BC in Alexandria. He made at least one trip to Jerusalem. His description of the trip is found in "On Providence", frag. II: "There is a city of Syria, on the sea shore, Ascalon by name: when I was there, at the time when I was on my journey towards the temple of my native land for the purpose of offering up prayers and sacrifices therein . . ." That is it. Nothing about Jerusalem. And there is no indication when he made that trip.
Also preserved are two fragments of Philo's description of the Essenes, a sect of Judaism associated perhaps with the Dead Sea Scrolls and the Qumran community and sometimes said by modern historians to be connected with John the Baptist. The fragments contain only descriptions of Essene life. There is no suggestion in the fragments of Philo visiting an Essene community on the Dead Sea, though there are hints of personal observations that may indicate that he did.
Philo also made at least one trip to Rome in 40 AD to argue before Roman Emperor Caligula for fair treatment of the Jewish community in Alexandria. That is all we know of Philo's life.
The question is whether we should expect Philo to have had knowledge of Jesus either from his personal acquaintance with Jerusalem or from his acquaintance with an early Christian community in Alexandria and whether he would have written about it if he had.
The answer is probably no. Why?
First, Philo was not an historian. He wrote nothing about Jerusalem that we have in our possession aside from his brief mention that he was going there on a spiritual pilgrimage. He was a philosopher and theologian not a travel writer or historian.
Second, if he had been there during the time of Jesus' teaching ministry would he have noticed Jesus? Maybe. but he might very well have seen Jesus (or John the Baptist) as an Essene. That is what it would have looked like to someone not well acquainted with Jesus but acquainted with the Essenes as Philo was. But there is no indication of either. So it is probable that Philo was not in Judea during the window of time of 29-33 AD.
Third, would he have written about Jesus from Alexandria? Probably not. Philo was interested in theology. Even if he had heard of Jesus or of his crucifixion, it would not have seemed like a very significant event. A lot of men were crucified in Jerusalem. There were even a lot of messiah's or would be messiahs, some about whom Josephus writes. And Philo writes of none of them. It was not his interest.
Finally, would Philo have encountered Christians in Alexandria? That is possible. But it is also unlikely. Followers of Jesus may not have gotten as far as Alexandria prior to 40 AD. And if they had, they would have been a very small obscure group of what would have looked then to be a Jewish sect. Christians at that time even saw themselves as a Jewish sect, though Messianic. If Philo had known about them, he would have considered them similar to others of the many Jewish sects in the city. But there is not one word from Philo about any of the Jewish sects in Alexandria. They were not his interest.
Botton line: Not finding anything about Jesus in Philo's writings is hardly surprising. It is most likely he knew nothing of Jesus or didn't care enough about Jesus to write about him. Philo's silence, therefore, is no argument for the non-existence of Jesus.