Can you work out the word? (My original puzzle)

Question

Help - the evil word splitter has been cleaving words in two with an evil curse! Fortunately, the word split in two can be reunited by saying its name correctly. But unfortunately, each half of the word can only talk about its own name and can only tell you what it is by a short riddle. Also, we don't know which half comes first or second. Hear what one poor word that has been split in two has got to say:

One half:

I have been a vice for many great thinkers over the ages to fully define exactly what I am.
I am the Good at times but also the Evil at others.
I have a twin sister: but I can't say her name although I can say her name rhymes with 'capacity'.
Currently, I am unfortunately missing a letter to complete my name: you can find this letter at the start of a smile.

The other half:

I am a youngster in the history of mankind.
My creator made me as a bond between land and sea.
Lately I am mainly the associate of vanity.
But numbers are my true friends.

Please help this poor word by finding out what it is!

Hint

For the top riddle: Plato, Aristotle and Kant. Bottom: You most likely own one Complete word: How do I pronounce borough?

Answer 1

I think the word must be

Phonetics

One half:

Et(h)ics

I have been a vice for many great thinkers over the ages to fully define exactly what I am.

Ethics - studied by great thinkers such as Plato, Aristotle and Kant.

I am the Good at times but also the Evil at others.

Ethics is a branch of philosophy that involves systematizing, defending, and recommending concepts of right and wrong conduct, "good" and "evil".

I have a twin sister: but I can't say her name although I can say her name rhymes with 'capacity'.

I think this may be either morality, which is a near-rhyme with "capacity", or perhaps veracity (truth).

Currently, I am unfortunately missing a letter to complete my name: you can find this letter at the start of a smile.

Etics is ethics without the h which is at the start of happiness.

The other half:

Phone

Note: This part had already been solved by LOTGP in their answer

I am a youngster in the history of mankind.

Phones are relatively new in the history of humanity.

My creator made me as a bond between land and sea.

The phone was designed for long-distance communication.

Lately I am mainly the associate of vanity.

With the dawn of the smartphone we have the concept of a "selfie" which can be shared instantly via social media.

But numbers are my true friends.

Phone numbers

Hint 3

Phonetics is the study and classification of speech sounds

Answer 2

I'm not 100% confident of this answer, but is the word:

Neophile?

First Half (Most likely correct):

phone

I am a youngster in the history of mankind.

Recently invented

My creator made me as a bond between land and sea.

Designed for long distance communication, as well as communication with ships

Lately I am mainly the associate of vanity.

Nowadays associated with social media and selfies, often seen as vain.

But numbers are my true friends.

Phone numbers are how you call people.

Second half (A bit more iffy):

life

I have been a vice for many great thinkers over the ages to fully define exactly what I am.

The concept of life has been a longstanding philosophical debate, and even now it doesn't have a specific definition.

I am the Good at times but also the Evil at others.

Life is what it is. It can be seen as either good or evil depending on what circumstances arise. Also The Good Life can refer to Eudaimonia, which is a greek word referring to happiness, which Aristotle discussed in his works.

I have a twin sister: but I can't say her name although I can say her name rhymes with 'capacity'.

This one could be many things, depending on how you interpret "twin sister". It could be vitality if you want them to mean similar things, or mortality/fatality if you want opposite meanings.

Currently, I am unfortunately missing a letter to complete my name: you can find this letter at the start of a smile.

Implies that a character must be removed from the word in order to get its current form. I ended up removing the f to get it to fit, but unfortunately can't justify it. (Maybe face?) Thus, we get lie

Thus, we end up with two word portions:

phone and lie

If we take anagrams of each and then mash them together, we end up with

Neophile

Answer 3

Don't think this is correct, as its technically still two words, but here goes nothing.

Is the answer:

Spider Monkey

PART1

I have been a vice for many great thinkers over the ages to fully define exactly what I am.

Pride, philosophy views on this vary and resulted in debate

I am the Good at times but also the Evil at others.

Pride, can be described as feeling confident and self-respect of ones self however can result in socially marginalising other groups.

I have a twin sister: but I can't say her name although I can say her name rhymes with 'capacity'.

Audacity: Aggressive, Bold, Proud (Pride)

Currently, I am unfortunately missing a letter to complete my name: you can find this letter at the start of a smile.

Adding an S onto Pride

From here

I've added an S onto Pride which I've rearranged as Spider

PART2

I am a youngster in the history of mankind.

The last common ancestor of humans are monkeys

My creator made me as a bond between land and sea.

Evolution of the monkey joins humans (land based) with fish (sea based)

Lately I am mainly the associate of vanity.

Not sure

But numbers are my true friends.

Monkeys like to live in numbers. Spider Monkeys live in troops and when different troops of spider monkeys meet, they hug each other to express greetings and avoid confrontation and potential aggression.