[kllrnohj]

Source 2 is Source 1 with most of the key systems replaced. They may have started with physics, but they didn't stop there. Many game engines are a collection of modules, Source included. So it becomes a fuzzy line when it becomes a "new" engine. Does replacing one module make a new engine or not? How about 2? 3?

And I very much do mean "replaced" there. Physics, since you mentioned that, was switched from Havok to the in-house developed Rubikon. And since Havok is a licensed middleware, they couldn't just bolt some new stuff on and call it theirs. That's going to be a full from scratch replacement.

Similarly the "UI module" was fully replaced, from the Flash-based Scaleform to Valve's in-house Panorama which is fairly similar to HTML5/CSS/JS. This module replacement was also "ported" to Source 1, and was implemented in CSGO as well. Which gets back to the lines between game engines "versions" are blurry.

[smileybarry]

Another nice example of that is the engine used in Ubisoft's Splinter Cell games. Technically it's Unreal Engine 2.5 (and some of the file formats are similar), but every single subsystem was replaced by now that only some of the tooling is still Unreal Engine 2.5 (which is arguably the important part to keep intact for game designers). Even the renderer isn't UE2.5 anymore.

[tecleandor]

It's the Engine of Theseus! ;)