diff --git a/spam-filter/core.scm b/spam-filter/core.scm
new file mode 100644
index 0000000..ee831d2
--- /dev/null
+++ b/spam-filter/core.scm
@@ -0,0 +1,124 @@
+(define-module (spam-filter core)
+  #:use-module (oop goops)
+  #:use-module (ice-9 match)
+  #:use-module ((srfi srfi-1) #:select (delete-duplicates fold any)))
+
+(define (get-words doc)
+  (delete-duplicates
+   (map string-downcase
+        (filter (lambda (s) (< 2 (string-length s) 20))
+                (string-tokenize doc)))))
+
+(define-class <classifier> ()
+  (get-features #:init-keyword #:get-features)
+  (feature-category-count #:init-thunk make-hash-table)
+  (category-count #:init-form '() #:accessor category-count)
+  (thresholds #:init-form '() #:accessor thresholds)
+  (weight #:init-value 1.0 #:init-keyword #:weight #:accessor weight)
+  (assumed-prob #:init-value 0.5 #:init-keyword #:assumed-prob #:accessor assumed-prob))
+
+(define-method (get-features (c <classifier>) doc)
+  ((slot-ref c 'get-features) doc))
+
+(define-method (set-threshold! (c <classifier>) cat t)
+  (set! (thresholds c) (assoc-set! (thresholds c) cat t)))
+
+(define-method (get-threshold (c <classifier>) cat)
+  (or (assoc-ref (thresholds c) cat) 1.0))
+
+(define-method (incf (c <classifier>) feature cat)
+  "Increase the count of a feature/category pair"
+  (let* ((fcc (slot-ref c 'feature-category-count))
+         (cc (hash-ref fcc feature '()))
+         (n (or (assoc-ref cc cat) 0.0)))
+    (hash-set! fcc feature (assoc-set! cc cat (1+ n)))))
+
+(define-method (incc (c <classifier>) cat)
+  "Increase the count of a category"
+  (let ((n (or (assq-ref (category-count c) cat) 0.0)))
+    (set! (category-count c) (assq-set! (category-count c) cat (1+ n)))))
+
+(define-method (fcount (c <classifier>) feature cat)
+  "The number of times a feature has appeared in a category"
+  (let* ((fcc (slot-ref c 'feature-category-count))
+         (cc (hash-ref fcc feature '())))
+    (or (assoc-ref cc cat) 0.0)))
+
+(define-method (catcount (c <classifier>) cat)
+  "The number of items in a category"
+  (or (assq-ref (category-count c) cat) 0.0))
+
+(define-method (totalcount (c <classifier>))
+  (fold + 0.0 (map cdr (category-count c))))
+
+(define-method (categories (c <classifier>))
+  (map car (category-count c)))
+
+(define-method (train (c <classifier>) doc cat)
+  (for-each (lambda (feature) (incf c feature cat))
+            (get-features c doc))
+  (incc c cat))
+
+(define-method (fprob (c <classifier>) f cat)
+  (if (zero? (catcount c cat))
+      0.0
+      (/ (fcount c f cat)
+         (catcount c cat))))
+
+(define-method (weighted-prob (c <classifier>) f cat)
+  (let ((basic-prob (fprob c f cat))
+        (totals (fold (lambda (cat accum) (+ accum (fcount c f cat))) 0.0 (categories c))))
+    (/ (+ (* (weight c) (assumed-prob c))
+          (* totals basic-prob))
+       (+ (weight c) totals))))
+
+(define-class <naive-bayes> (<classifier>))
+
+(define-method (doc-prob (c <naive-bayes>) doc cat)
+  (fold (lambda (feature p) (* p (weighted-prob c feature cat)))
+        1.0
+        (get-features c doc)))
+
+(define-method (prob (c <naive-bayes>) doc cat)
+  (let ((cat-prob (/ (catcount c cat) (totalcount c))))
+    (* cat-prob (doc-prob c doc cat))))
+
+(define-method (classify (c <naive-bayes>) doc)
+  (define max 0.0)
+  (define probs '())
+  (define best #f)
+  (for-each (lambda (cat)
+              (let ((p (prob c doc cat)))
+                (set! probs (assq-set! probs cat p))
+                (when (> p max)
+                  (set! max p)
+                  (set! best cat))))
+            (categories c))
+  ;; Make sure the probabilty exceeds threshold*next best
+  (display probs) (newline)
+  (if (any (match-lambda ((k . v)
+                          (and (not (eq? k best))
+                               (> (* (get-threshold c best) v) max))))
+           probs)
+      #f
+      best))
+
+(define-method (sample-train (c <classifier>))
+  (train c "Nobody owns the water." 'ham)
+  (train c "the quick rabbit jumps fences" 'ham)
+  (train c "buy pharmaceuticals now" 'spam)
+  (train c "make quick money at the online casino" 'spam)
+  (train c "the quick brown fox jumps" 'ham))
+
+(define-method (sample-train-n-times (c <classifier>) n)
+  (let loop ((n n))
+    (when (> n 0)
+      (sample-train c)
+      (loop (1- n)))))
+
+(define (make-test-classifier)
+  (let ((c (make <naive-bayes> #:get-features get-words)))
+    (set-threshold! c 'spam 3.0)
+    (set-threshold! c 'ham 1.0)
+    (sample-train c)
+    c))