The philo stone and crystalline flask are there to help you farm in lane to get the gear you need.

The philo stone is a must because it will help mitigate harass against you and allow you to harass back while providing an additional income.

The gold provides .5 gold each second and costs 700g total so it pays for itself in 1400 seconds or 23.33 minutes and is pure profit beyond that point. This coupled with the ability to safely last hit with skills getting CS that you'd otherwise miss makes it worth picking up on both of these champions.

The same is true for the flask. It is also very cost effective.

After you get a philo, you build 4 things. % armor pen, flat armor pen, AD, and CDR.

CDR allows more skill spamming while the penetration allows you to abuse the skill's base damage values. And the AD's value is boosted by both CDR and Penetration.

There are currently 3 things that provide % armor penetration available to champions outside their kit.

The Black Cleaver provides up to 30% armor penetration. The last whisper provides 35% armor penetration and the offense mastery weapon expertise provides 8% armor penetration.

By simply building 2 items and running 9 offense mastery points it is possible to penetrate:

.7 x .65 x .92 x 100 = 41.86 (% armor remains)

100-41.86= 58.14 (% armor penetrated)

This is absurd... considering the cost. BC only costs 2865g while the last whisper costs 2290g for a total of 5155g.

Then on top of this, the BC has 15 armor penetration and most people running this build run armor penetration runes as well for an additional 16.6 armor pen and most run enough offense masteries to get sunder for 6 more.

So with no additional armor pen that is 15+16+6=37.6 flat armor pen.

In order to not get hit for TRUE DAMAGE the enemy needs to have more than:

37.6 = X * .4186 => X= 89.82 armor

What makes it worse is that once you have built both a last whisper and a BC, each point of additional armor penetration is cheaper to buy than each point of effective armor.

Let me show you what I mean.

A long sword costs 400g and gives 10 AD so each AD is worth roughly 40g.

To upgrade 2 long swords to a brutalizer the patter cost is 537 gold. Your get 5 AD so subtract 5*4o = 200 gold from this to cover the bonus AD and even if we say the CDR is no value the 15 armor pen is worth 337g for a ratio of 22.5g per armor pen.

Now look at armor. A chain vest costs 720g and gives 40 armor. But only 41.89% of that armor is effective against you once you have a BC and a Last whisper. So really it only provides 16.756 effective armor. So the cost of 1 effective armor is 43g per effective armor.

THAT IS DOUBLE THE COST OF A POINT OF ARMOR PEN.

So now even specifically building armor and only armor against an AD champion, they can shred your armor more effectively than you can build it...

HOW IS THAT BALANCED? It's not.

Sure you can give up the inevitable losing race against armor pen and build hp instead...but then again with the addition of all the % hp damage items that is very easily countered in similar matter...

If they stop building armor and start building HP against you, build a Blade of the ruined king...it will decimate them...