On the nature of the wavefunction. A quantum system is described by a state vector |ψ⟩ living in a Hilbert space ℋ. Its evolution in time obeys the Schrödinger equation, iħ ∂|ψ(t)⟩/∂t = Ĥ|ψ(t)⟩, where Ĥ is the Hamiltonian, the operator that generates translations in time. For a single particle of mass m in a potential V(x), Ĥ = p̂²/2m + V(x̂), and in the position representation the equation reads iħ ∂ψ/∂t = (−ħ²/2m)∇²ψ + V(x)ψ. The solution is unitary, |ψ(t)⟩ = e^(−iĤt/ħ)|ψ(0)⟩, so information is never lost while the system is left alone. What we call a particle is a pattern of amplitude spread through space, and only the act of measurement asks it to choose a place. The Born rule gives the probability of finding it at x as P(x,t) = |ψ(x,t)|², with ∫|ψ|² dx = 1. Every observable is a Hermitian operator Â, its possible results are the eigenvalues a in Â|a⟩ = a|a⟩, and the expected value is ⟨Â⟩ = ⟨ψ|Â|ψ⟩. Position and momentum do not commute, [x̂, p̂] = iħ, and from this single line follows the uncertainty relation σₓσₚ ≥ ħ/2. The same structure binds energy and time, ΔE Δt ≳ ħ/2, which is why a state that lives forever has a perfectly sharp energy and a state that changes quickly carries a blur of energies with it. Superposition is the rule, not the exception: if |a⟩ and |b⟩ are allowed states then α|a⟩ + β|b⟩ is allowed as well, with |α|² + |β|² = 1. Two particles that have interacted can share one state that belongs to neither of them alone, |Φ⁺⟩ = (|00⟩ + |11⟩)/√2. Measure one and the other answers, however far apart they are. Bell showed that any theory of hidden local causes must satisfy |S| ≤ 2 in the CHSH arrangement, while quantum mechanics predicts and experiment confirms values up to 2√2. Feynman rewrote the whole story as a sum over histories: the amplitude to go from a to b is ⟨x_b|e^(−iĤt/ħ)|x_a⟩ = ∫𝒟x e^(iS[x]/ħ), every possible path taken at once, each weighted by the phase of its action S = ∫L dt. The classical path is simply the one where the phases agree, δS = 0. For the electron moving near the speed of light Dirac wrote (iγ^μ∂_μ − mc/ħ)ψ = 0, and antimatter fell out of the equation before anyone had seen it. When the whole universe is treated as one quantum system, the Wheeler and DeWitt equation reads Ĥ|Ψ⟩ = 0. There is no t in it. The wavefunction of everything does not evolve; time appears only inside it, as a relation between one part of the universe and another, a clock read against a world. Decoherence explains why we never see a cat both alive and dead: the environment records which branch is which, the off diagonal terms of the density matrix ρ = Σ pᵢ|ψᵢ⟩⟨ψᵢ| fade as e^(−t/τ_D), and a single history is left standing. Below the Planck length, ℓ_P = √(ħG/c³) ≈ 1.6 × 10⁻³⁵ m, space itself may stop being smooth. Entropy and area are tied together at the horizon of a black hole, S = k_B c³A/4Għ, as if the information inside a volume were written on its surface. Perhaps the world is a page, and depth is only the way we read it.